THE COMPONENTS OF A CEMENTED COMBINATION LENS. 15 



is a doublet or a triplet. Next find out the nature of the 

 exterior surfaces, and write them down, e.g., plano-convex 

 doublet. This means that the combination is composed of two 

 lenses, and that one of the exterior surfaces is convex and the 

 other plane. Now write down the reflections as they come, 

 beginning at the side next the window, underlining the reflec- 

 tion from the first surface, and putting the reflection from the 

 cemented surface in parenthesis. In writing these down I use 

 the following abbreviations : — e for erect, i for inverted, s for 

 small, I for large, and L for very large. It is a good plan to 

 draw the lens by representing, first the exterior surfaces only, 

 and then filling in the cemented surfaces, according to the 

 reflections you obtain. It is absolutely necessary that the 

 character of the reflections from both sides of the combination 

 should be ascertained, as it is impossible to discover the con- 

 struction of the combination from one set of reflections. 



When the images are large, it is as well to look at the reflec- 

 tion of the bar across a window ; the knob of the hasp showing 

 if the image is erect or inverted. 



The images from small lenses require to be examined by a 

 magnifying glass. 



One word of caution, and that is, until one is practised in 

 picking up these faint images the very large faint ones are apt 

 to be overlooked. Until one is familiar with the manner of 

 holding a lens only a faint blue tinge will be seen over the 

 glass, but after a little practice a distinct image of the window 

 bar will be obtained. 



The following are a few examples : — 



A plano-convex doublet : — 



Convex side, se (si) li. 



Plane side, le (se) si. 



It therefore consists of a biconvex and a plano-concave. 



The next two form an interesting pair. 



A converging meniscus doublet : — 



Convex side, se (si) li. 



Concave side, li (se) si. 



This, therefore, consists of a biconvex, and a biconcave. 



Another lens of the same exterior form : — 



Convex side, se (se) li. 



Concave side, li (si) si. 



