292 HOW TO SEE WITH THE MICROSCOPE. 



creased. This latter item is to be a constant study ; 

 bear in mind this one broad rule, that to use a wide- 

 angled glass well is to work with the least obliquity 

 possible, and when over easy tests, with more central 

 illumination that can be effectively obtained with nar- 

 row apertured objectives. In short the employment of 

 the wide apertures points to the use of centrally dis- 

 posed light ; this may be a novel doctrine to many, but 

 not at all new to the author. 



Strive then to work with the most central illumina- 

 tion possible ; get rid of all shadows not hidispensable ; 

 as a rule where the markings are dependent on thick- 

 ness of structure, the greater this difference of thick- 

 ness, i. e., the more prominent the markings, the less 

 will be the obliquity required; conversely the thinner 

 and the fainter the markings the more obliquity is 

 called for. Now the smallest shells of the Isle of 

 Shoals are extremely thin, the striae are not so very fine, 

 probably not measuring closer than 85 in .001 English 

 inch, but they are so thin and so fine as to beat out any 

 object-glasses, excepting those of the higher apertures. 

 And even with these the mirror will have to stand (ar- 

 tificial illumination being used as directed) at least (50 

 to 65 from axis; a fine one-tenth ought to show any 

 and all of them, either with the one inch or one-fourth 

 inch ocular. A similar one-sixth will require the half- 

 inch. 



Now, in the study of the last four or five lessons, the 

 student is to constantly endeavor to improve himself in 

 the adjustment of his objective. The broad land-marks 



