8f, SELECTION AND USE 



seen those who could not distinguish lines that were visible to 

 others, and we have also met those to whom an objective, in 

 which the chromatic errors were very obvious, seemed to be 

 perfect. This probably arose from a kind of color blind- 

 ness. We have also met eyes which distorted objects, and 

 those which saw fringes of color round objects viewed through 

 an objective of generally recognized excellence. 



Attempts have been made to get rid of the errors arising 

 from personal peculiarities (or what may perhaps be called the 

 "personal equation") by employing photography, it being 

 assumed that if a lens will give an image which can be photo- 

 graphed, it must give an image that may be seen, and that 

 whatever is photographed must of necessity be a real image. 

 But from the known fact that the foci of the chemical and 

 visual rays do not coincide, and that the corrections required 

 in the one case are not those calculated to give the best results 

 in the other, we have little faith in photography as the best 

 test of the excellence of an objective, except, of course, in those 

 cases where photographic work is the chief purpose in view. 

 Lenses intended to transmit an image to the eye must be tested 

 by the eye, and if certain eyes show peculiarities not possessed by 

 the average eye, then lenses must be corrected specially for them. 



It is scarcely necessary to say that when an objective is put 

 upon its trial, the stand and means of illumination ought to be 

 such as will do it justice. The best stand in the world cannot 

 make a good objective out of a poor one, but a poor stand will 

 give poor results even with the best objective. The eye-pieces 

 also should be of good quality, and if an objective, which the 

 microscopist has reason to believe is a good one, fails, it should 

 not be condemned until it has been tried with eye-pieces either 

 by the same maker, or of a known standard of excellence. And 

 we must also remember that it is not sufficient to examine an 

 objective in combination with a shallow eye-piece, or one giving 

 a low magnifying power. An objective may perform very well 

 if used with low eye-pieces, and utterly fail when a higher 

 power is applied. Most makers of objectives test their glasses 

 under eye-pieces of very high power a quarter and even an 

 eighth of an inch focus, or what would be equivalent to H or K 

 on the usual scale. 



