Not all things are equal in a natural history collection. The Canadian Museum of Nature makes an effort to classify what’s in the collections into one of five categories, with category one containing the most unique, scientifically important specimens, and onward to five, which is for specimens that are useful, but way less important to protect.
When scientists describe a new species of plant or animal, they typically do so based on a single specimen. That particular specimen is then conserved in a museum collection for all time as the best source of data for that species. (There are currently about 1.9 million named species on Earth). That unique, non-replaceable specimen is called the type. The type specimen is the item to which all other studies on that species are compared. When the description of a new species is published, the type specimen is given a unique identifier so others can find it for their studies.
The natural-history-museum business has operated in this manner for centuries, and it works just fine, especially if you are working on things that are visible to the naked eye. If you are doing a study that revises how we classify groups of species (which happens regularly), you will need to consult the type material: you contact a curator to have access to it, lay it out on a table and away you go.
But not all species are built that way. Even though scientists find many new species of megafauna each year, most of the new discoveries, and in fact most of the species on our planet, are really small. Some can be seen only with a microscope.
Some species of algae (protists) are part of this microscopic world. Even though they are invisible to us, these microbes (such as diatoms) are the foundation of the aquatic food web. All other forms of life rely upon them, directly or otherwise—from the smallest crustaceans to the largest whales.
Microscopic specimens come into museum collections on a regular basis and are prepared on glass slides for analysis. When a new species is discovered, it is described in the same manner as other species, typically from a single specimen. And that creates a special challenge. That single specimen is resting on a glass slide amongst thousands of other specimens. How does a museum curator keep track of that single, important, type specimen?
Some curators use a finder slide with a grid of lines and numbers to locate the type specimen. After a specimen is found on a sample slide, the sample slide is removed with care to keep all the settings on the microscope the same. The sample slide is replaced with the finder slide, and the position of the specimen in question is noted relative to the grid on the finder slide. That reference becomes the code that creates value on the sample slide forevermore. But you must never, ever lose the finder slide or else the entire collection on glass slides becomes a mystery. The use of a finder slide is an established method at the Alfred Wegener Institute in Germany.
Specimens on glass slides are covered with a very thin slip of glass. Another method to find individual microbes is to etch a circle on the glass cover slip that encircles the specimen. This is done with a tiny diamond. The Academy of Natural Sciences in Philadelphia, U.S.A., uses this technique.
A third method is to create a starting position on the slide and make a road map to the type specimen that is recorded in millimetres, going up or down and across.
With the aid of photography to record the field of view, in combination with any of these methods, it is relatively easy to get back to a spot that you want to remember.
At the Canadian Museum of Nature, we use all three techniques to remember where we have stored microscopic specimens, although the first two are the most common. The museum also has well over a thousand type specimens in its other collections of plants, animals, fossils and minerals.