MEDIUM-POWER PHOTO-MICROGRAPHY 



129 



produced hy change in the N. A. of the system that the attention of the reader must 

 be directed. A word in explanation may here be given. To say that this variation 

 is inversely as the N. A. is only another way of expressing the fact that it varies 

 directly as the reciprocal of N. A., and as the reciprocal of N. A. is expressed by (^-^ ) 

 a ready method of comparing the penetrating power of the different apochromatic 

 powers used is afforded by a study of the accompanying table. 



For N. A. o"3o ........ 3"333 



o"65 1-538 



o'95 i'o53 



n i'4o 0714 



It is quite evident that the i-inch gives the greatest depth of focus, and that 

 the ^ and yV, each of i"40, give the least. We can now see which of the six combina- 

 tions already mentioned will best suit the conditions enumerated in a, (3, and y. If 

 the requirements are as propounded in a, where we desire the highest possible detail 

 irrespective of depth of focus, then, small as the field would be in No. 5 or No. 6, 

 they will be the combinations best suited for the purpose, because the details 

 furnished will be unquestionably greater ; but if (y), where merely the general 

 view of everything is desired every v/here, and detail a secondary object, then we 

 should choose No. i or No. 2. But granted that neither the extreme of (a) nor that 

 of (y) suit the wishes of the photographer, then No. 3 or No. 4 will be the arrange- 

 ment we should recommend for {(3). Here, then, is a great opportunity for th© 

 operator to display his "individuality," for he must make his choice, and we counsel 

 those whose experience is limited to try several of the combinations before coming to 

 a definite conclusion. 



To resume : having now focussed with the greatest care, we expose — what we 

 think, from the considerations laid down, to be the correct amount — and develop and 

 fix in the ordinary fashion as already described. 



Another good specimen to practice upon is the loot of a very small garden spider. 

 The substage diaphragm may here require small closure, and the Davis diaphragm 

 just the smallest touch to gain depth of focus. It is shown in Fig. 9, Plate HI. 



The next class of slide which may be taken with the half-inch is a diatom. It 

 serves to illustrate a different type of object. Before purchasing one — let us say an 

 Arachnoidiscus Ehrenberghii (Fig. 5, Plate III.) — the photographer must assure him- 

 self it is mounted perfectly flat on the cover-glass and in a highly refractive medium. 

 We know of no mounter of diatoms in the United Kingdom that can surpass Mr. 



E 



