Ants and their bad day
To make the slitlamp exam a thing you “get” let me pose a question to you: Have you ever used the sun and a magnifying glass in nefarious ways? Have you ever burned leaves of “popped” ants. Once when I was at the Wills Eye Course Dr. David Guyton brought this up during the optics review. He asked the large group of us if we knew why ants made that popping noise when you used the sun and a magnifying glass to heat them up. I think we all laughed that quiet nostalgic laugh.
Well, he told us the answer. He said that the heat makes their insides boil and eventually their exoskeleton ruptures with a pop! Somehow the audience found this satisfying, but maybe the ants don’t. I am not an ant, but I guess that is one of the ways an ant can have a bad day.
Now how do you do this?
Well, it is not difficult. Look at YouTube and you will see kids posting and narrating this stuff. 1 So if you want the ant to pop or the leaf to burn you must make the focal point as small as possible. If you are too close or too far away the ant gets warm, but he does not pop.
A more normal use of that magnifying glass
Usually a magnifying glass magnifies. You would probably learn more by looking at the ant with that instrument rather than making him poof.
A slit lamp is very much like this magnifying glass with one major exception. You cannot set your patients on fire with the light though they may feel it is bright enough to pop their eye.
The point though is to put the focal point of the slit lamp on the various parts of the eye to make clinical observations. Look at the two dimensional view below and relate it to the three dimensional image above.
This two dimensional view is how magnifying glasses work AND it is how the slit lamp works. There is only a single point where the slit lamp provides sharp focus. That does not mean you will not see other structures, you will. Those other structures though will be blurry to some extent. The closer those other structures are to the focal length the more distinct they will be. This is important to understand because by that you will know which direction to move the slit lamp in order to visualize other structures. Look, now, at the figure below and click through it. Notice the manner in which the sharp focus “descends” into the eye.
Slitlamp exam schematic
As the slit lamp moves closer to the eye the structure in focus will simultaneously be deeper. Using the slit lamp alone you will routinely be able to see structures as internal as the anterior vitreous. In most cases you will need to use a condensing lens, e.g. the 90D lens to see deeper structures, but that is for another lesson.