The Dissecting Needle: The Microscopist’s Utility Player

In 1974, the first McCrone Microscopes & Accessories product catalog was published. It was a textbook of sorts titled “Techniques, Instruments, and Accessories for Microanalysts — A User’s Manual”. The catalog featured many products selected from John Gustav Delly’s microscopy bench, the items he used in his everyday work. The “User’s Manual” contains common as well as difficult to find items with product descriptions aimed to aid the particle microscopist. Among the many products pictured is the common dissecting needle accompanied by Delly’s description of its uncommon uses, adding greater depth to its utility as a microscopy tool.

Let’s take a closer look at some of the uncommon microscopical uses of the dissecting needle.

Bursting Your Bubbles

Before the publication of the McCrone “User’s Manual”, we find evidence of microscopists using the dissecting needle for other purposes beyond dissection. Such is the case in John Quekett’s book “A Practical Treatise On the Use of the Microscope” published in 1855. Quekett describes some not-so-common uses for the dissecting needle:

“For the purpose of destroying air bubbles, or moving about the preparations after they have been placed in balsam, and for various other uses, a needle fastened into a handle of wood is the answer… the broad end of the needle will serve for pressing down the glass cover…By this instrument, preparations are adjusted to their proper situations in the balsam, air bubbles are drawn away from the neighbourhood of the object; or, if necessary, they may be burst by touching them with the point when slightly warmed”

Warming of the needle to destroy an unwanted air bubble or two in Canada balsam preparation is a nice touch. This technique works with modern mounting media, such as MeltMount.

Poking and Prodding

Emile Monnin Chamot, in his 1922 book “The Microscopy of Small Arms Primers”, describes how the use the dissecting needle under low magnification can reveal information about the physical properties of firearms primer pellets:

“The texture of the pellet may be determined by pressing a needle into them (Fig 4). Hard, well-formed compressed pellets offer considerable resistance to the entrance of a needle…The largest fragments are next tested with the needle to ascertain the texture…a pellet of loose texture…was badly ruptured…”

Probing the sample with a dissecting needle provides useful information prior to a microscopical analysis. I remember being shown the importance of “teasing apart” an insulation sample to see if any fibers clung to the forceps and dissecting needle, indicating the possible presence of chrysotile asbestos. Not a positive identification, but gets you thinking about the sample before its microscopical examination.

The User’s Manual Approach to the Dissecting Needle

In the McCrone “User’s Manual”, Delly begins the dissecting needle description by telling the reader how important this tool is for the particle microscopist and how the one he uses has been customized:

“The Dissecting Needle…is truly a versatile and indispensable instrument. This is a biological type of needle which we have shortened to achieve just the right amount of flexibility for its multitude of uses.”

After shortening, Delly goes on to describe the dissecting needle’s not-so-common uses for the particle microscopist, ranging from particle collection techniques to observing their physical properties. Let’s begin with particle collection.

Making the Drop

By simply dipping the dissecting needle into a balsam bottle containing a mounting medium like MeltMount and pulling the needle straight out, a small droplet will remain on the end of the needle. Here is how Delly describes it:

“…the technique consists of dipping the needle…so as to form a globule on the end, which is then allowed to cool. When cool, the sticky droplet is used to pick up particles from a substrate. The particle-laden droplet is then melted on a warmed slide and covered with a slip.”

Getting the drop to remain on the end of the needle is a matter of adjusting the temperature of the MeltMount. If too hot, most of the MeltMount drop will fall back into the balsam jar; if too cool, you will end up with a stringy mess of MeltMount around the opening of the balsam bottle.

This technique is described later in Section III of the “User’s Manual”, and applied to the collection of particles from a membrane filter:

“If the sample is small, say a few particles on a membrane filter, a dissecting needle may be carefully dipped…to form a small globule on the end of the needle. The globule can be used to sweep the filter. All particles will adhere to the…globule…then be melted on a slide on a hotplate, and the cover glass can be added later.”

To save time, I decided to pre-make a few dissecting needles with a drop of MeltMount to have on hand for later use. After storing the needles for a few days, I noticed the drops of MeltMount began to move. When stored upright, the droplet ran down the side of the needle, if stored facing down, the droplet eventually fell off the needle.

I found the best practice, as suggested by Delly, is to use the MeltMount droplet on the end of the needle a few minutes after being formed.

Getting the Scoop

The dissecting needle can be used to transfer material from a vial onto a microscope slide, like using a micro-spatula. From the “User’s Manual”:

“The dissecting needle can also be used as a microspatula in removing samples from vials. Care must be exercised here, however, as the particle size fraction removed will be selective, and not characteristic of the entire sample—the smaller particles being favored.”

Realizing you may create a particle size bias using this method is good to know!

Magnetic Fields of View

The dissecting needle can become easily magnetizing by stroking the needle across a magnet. This is a great way to separate magnetic particles from non-magnetic particles, and to detect magnetic particles in a liquid preparation sample. From the “User’s Manual”:

“Another use for the needle is in separating magnetic from nonmagnetic fractions in a sample. Simply stroke the needle across the end of the magnetic end of a pocket scriber…and “swish” the needle around in your sample. On withdrawing it, you will remove most if not all of the magnetic particle.”

Once you have made your dissecting needle magnetic, wave it above a liquid microscopical preparation and watch for any magnetic particles “swimming” in the direction of the needle. From the “User’s Manual”:

“The magnetized needle may also be waved over an individual particle in a slide preparation while viewing it with the compound microscope…Magnetic particles in a fluid medium will respond like compass needles to movements of the magnetized needle.”

Below is an example of what Delly is talking about regarding magnetic particles responding to a magnetized dissecting needle. The first photomicrograph shows a randomized field of view of sand particles under reflected light illumination. The second photomicrograph illustrates how some of the magnetic sand particles have migrated through the liquid mounting medium to the upper left corner of the field of view, circled in yellow.

Under Pressure

Like Chamot’s technique of probing firearms primers, applying pressure to a coverglass with a dissecting needle and observing how the particles react can give you additional information about your sample. Do the particles crumble or break open? Are they hollow? Do they spring back into shape when the pressure from the dissecting needle is released? These are some of the questions Delly assures the reader can answer with a little help from the dissecting needle. From the “User’s Manual”:

“The reaction of particles on being depressed can give you valuable information. If while watching a wormlike particle, you depress the cover slip with the needle and the particle squoshes and stays flattened when the needle pressure is released, you may have a grease, or mucilage, or tarry substance.”

The two photomicrographs below illustrate a field of view of bitumen from an oil sand sample, which usually appears as a sticky black liquid. The first photomicrograph below shows bitumen mounted in water. The second image is the same field of view after the application of light pressure from the dissecting needle and then released. The bitumen globule remains flat.

Similarly, when pressure is applied using the dissecting needle to a group of microscopic glass balloons, pictured below, some of the thinner-walled light brown balloons crack open, indicating they are brittle and hollow. From the “User’s Manual”:

“It may answer the question of whether that opaque, black sphere is hollow or not, or whether that flaky particle is brittle or not.”

For elastomeric samples, when applying a bit of pressure to the coverglass with the dissecting needle, some particles become flat, and then return to their original shape after the pressure is relieved. From the “User’s Manual”:

“If, on the other hand, after flattening, the particle springs back to the original shape, it is most likely an elastomer. In any case, you have learned one more characteristic of an unknown particle.”

Below are three photomicrographs of particles from a rubber stopper: the first with no pressure added to the coverglass, then adding pressure with a dissecting needle, and finally without pressure. Without pressure from the dissecting needle, the rubber particles return to their original shape.

What About Permanent Mounts?

All the examples illustrated up to this point have been prepared in liquid mounts. But what if you have a permanent preparation—particles mounted in MeltMount, for instance? Since MeltMount is a thermoplastic resin, the preparation can be reheated on a hotplate and will become liquid again. By quickly removing the sample from the hotplate and placing it on the microscope stage, these same techniques can be applied before the MeltMount solidifies again. From the “User’s Manual”:

“…you will have to place the slide on a micro hot plate for a few seconds to make the mountant fluid (don’t forget to swing out the top lens of the condenser when returning the slide to the microscope stage! If you don’t, you are apt to introduce strain in the lens.)”

I was surprised at how long the thin layer of MeltMount in my preparation remained liquid after being removed from the hot plate.

When You Needle Little Help

Tools like the dissecting needle, with its many elegant uses, can elevate your game as a microscopist. The dissecting needle’s understated brilliance lies in its flexibility, making it an indispensable part of a microscopist’s toolkit. Your lab probably has at least one dissecting needle kicking around. With your new-found appreciation of the dissecting needle’s uses, start putting it into your microscopy lineup to help you hit your next microscopical home run.