13 



On a Method of Finding the General Character of the 

 Components of a Cemented Combination Lens. 



By E. M. Nelson. 



(Read November 26th, 1S86.J 



Some microscopists take no special interest in their lenses, 

 beyond occasionally wiping the front, and brushing the dust, 

 when it gets too thick, with a camel's-hair pencil from the 

 back. 



Others unscrew the combinations to clean them, and like to 

 know whether they are composed of two, three, or four sets of 

 lenses. 



Some, again, know all about the components of each com- 

 bination, and some wish they did. It is to this last class of 

 inquirers that this paper is directed. 



Let me first say that there is only one way of finding out the 

 exact composition of a lens, and that is by taking down every 

 combination, uncementing every lens, measuring the exact cur- 

 vature, and the refractive and dispersive power of the glass of 

 which it is made. 



It will be admitted, however, that it is very useful to know 

 whether a combination consists of two, or three lenses, and if 

 those are biconvex, biconcave, plano-convex, meniscus, &c. To 

 find such information without uncementing a combination is the 

 scope of this paper. 



The method I employ, is simply the consideration of the 

 reflected images from the surfaces of the glass. Take the plain 

 mirror of your microscope in your hand, and examine the reflec- 

 tion of a window, notice that it is an erect image, and that 

 when you move the mirror in a certain way the image appears 

 to come towards you. 



Now look at the concave side, the image is inverted, and when 

 the mirror is moved in the same direction as before the image 

 goes away from you. 



A convex mirror behaves as a plain mirror, there being only 

 this difference, that the greater the convexity the smaller is the 



