M. A. AIXSLIE ON AN ADDITION TO THE OBJECTIVE. 569 



advantages possessed by objectives of medium power over those 

 of very high power. 



It will be seen then, I think, that this device places a con- 

 siderable power of correcting for cover-glasses of abnormal 

 thickness in the hands of the microscopist, especially with the 

 miserable 50 mm. range of the Continental draw-tube ; and it 

 now remains to be seen what the effect of this additional lens is 

 on the power, N.A., and corrections, spherical and chromatic, 

 of the objective. 



Firstly, as regards the power. If the additional lens could in 

 practice be fitted in the ; ' upper focal plane " of the objective 

 that is to say, in the plane in which a pencil of parallel rays 

 entering it from below would come to a focus there would be 

 no alteration of power. But in most objectives in all those of 

 high power, in fact this upper focal plane is not far behind 

 the front lens, and therefore inaccessible. So we have to put 

 up with a certain increase of power in the case of the concave 

 lens, and a decrease in the case of the convex ; but if the power 

 of the additional lens does not exceed 4 diopters either way, i.e. 

 if its focal length is not less than 10 in., the alteration of power 

 is not serious. It is an advantage to have the additional lens as 

 near as possible to the back lens of the objective, but if there 

 is any difficulty in fitting it there, it does very well to place it 

 on the nosepiece. 



The effect on the working distance of the objective is not 

 serious : the concave lens increases the working distance, and 

 the convex lens diminishes it ; but since the former is used in 

 the case of thick cover- glasses, and the latter in the case of 

 thin, it will be seen that the change is in the right direction, so 

 that this point need not worry us. 



Similarly the effect on the N.A. is not great. The convex 

 lens somewhat diminishes it, and the concave lens may (it does 

 not always) slightly increase it ; but the change is not great, 

 and for most purposes unimportant. 



It is of more importance to inquire what effect, if any, the 

 introduction of the additional lens has on the spherical and chro- 

 matic corrections. To take the former, it so happens that the 

 spherical aberration introduced by the additional lens slightly 

 counteracts its effect in producing the result desired ; a simple 

 uncorrected lens does not quite produce the full theoretical effect 



