1899] MICROSCOPICAL JOURNAL. 297 



— which gives the better image the achromatic or the 

 apochromatic ? By the tutored eye small differences in 

 definition, powers of resolution, blackness of shadows, 

 flatness of field, etc., are all at once easily recognized; 

 but to the less experienced it becomes a tax of much greater 

 proportions and often necessitates a regular training of 

 the eye to thoroughly appreciate. The same remark of 

 course applies to the comparison of photographs produced 

 by the different lenses — unless you know what to look 

 for and how to look for it. It is the aim then of this ar- 

 ticle to assist those less acquainted with the subject to do 

 this with a little practice for themselves. 



•Before however approaching the actual subject itself, it 

 is necessary first to touch upon the theoretical difference 

 which underlies the manufacture and construction of 

 these different classes of objectives, not from a mathe- 

 matical point of view, for that is not within the scope of 

 this paper, but in what it is hoped may be considered 

 a practical manner. 



Probably every reader of this journal knows full well 

 what is meant by the word spectrum — the rainbow effect 

 that is produced by a beam of light passing through a 

 prism of glass. Such an experiment most of us have 

 been the unconscious witnesses of when we noticed the 

 colors produced by the sun shining on one of the pendants 

 of the glass lustres which often used to adorn old-fash- 

 ioned mantlepieces. The actual path of the ray of light 

 passing through the prism is in exageration, shown in 

 text-book figures, which everyone must recognize who 

 has even glanced at a textbook on light. It will be seen, 

 first, that the im-merging beam of white light is broken 

 up into several e-merging rays which are now all of dif- 

 ferent colors; and secondly, that these emergent pencils, 

 although their edges are continuous one with another, do 

 not overlie each other but appear fairly well separated, 

 the violet being the most bent, then the blue, next the 



