Otf THE MICROSCOPE. 89 



more rapidly than with glasses that require greater care and 

 skill. The same is true of the elementary studies of the bot- 

 anist and histologist, carried on under the guidance of a com- 

 petent teacher. And as in all such cases it is easy to find out 

 the special thickness of glass for which the object-glass has 

 been corrected, and to provide a supply of the proper thickness, 

 the absence of a means of adjustment for cover thickness is not 

 very important. But for all the higher class of studies, good 

 glasses, with well-made adjustments for thickness of cover, are 

 indispensable.* 



Objectives of very low angular aperture, are, however, to be 

 equally avoided. There is a want of light, and an indistinct- 

 ness which renders them worthless. It is generally said that 

 the superiority of large angles is most marked in the objectives 

 of high power, and that for low powers the common objectives 

 do very well. In our judgment, however, the superiority of 

 the low powers is quite as marked as that of the higher ones, 

 and much more available to the beginner. It is true that the 

 superiority of a well made one-sixth of high angle, over any 

 triplet of whatever focal length, is immeasurable, but at the 

 same time it is equally true that the view of an opaque object 

 seen through an inch-and-a-half objective, carefully corrected, is 

 as much superior to the same as seen through a common triplet, 

 as it is possible to imagine. We have now before us a speci- 

 men of bone of very open structure, mounted as an opaque 

 object. Seen through a first class inch-and-a-half objective, it 

 presents almost a stereoscopic appearance, and the entire struc- 

 ture is easily made out. The view afforded by a very fair French 

 triplet (No. 0) is so markedly inferior, that any person who 

 should see the two would never again use a cheap objective, if 

 he could afford to get a good one. Moreover, the objection 

 which we have just urged against objectives of high power, and 



* 



*It is not long since a professional maker of microscopes, and one who 

 geem to stand high in the favor of the medical profession, tried to per- 

 suade the author that the covering-glass exercised no influence on the 

 action of the objective, and that a non-adjusting glass could be made 

 with as great a resolving power, as one constructed so as to adjust for 

 different thicknesses of covering glass ! To such men, a famous micro- 

 scopist used to apply the term " shopticians," and they deserve it. 



