The Microscope. 



and he should see that they are tested on some histo- 

 logical object, and not on diatoms, as the wide angle 

 necessary for resolving test diatomaceae are the reverse 

 of useful to the young Histologist. The most important 

 glass for the student is the high power, and it is neces- 

 sary that this should be a good one. 



High Powers. The best high powers are made by 

 Messrs. Powell and Lealand, they are of course expen- 

 sive but pay well in the end; they are made with 

 correction collars and are more adapted to large stands 

 with mechanical stage. Zeiss' E = j- and F = ^ are 

 very good glasses, and well adapted to the students' 

 stand and are not expensive ; they have no correction 

 collar, and the cover glasses should be measured to get 

 the best effect ; -006 will do very well for D and E, and 

 004 for F, the same applies to Mr. Crouch's which is 

 corrected to a cover glass of -006. 



Oil Immersion Lenses. These glasses are taking the 

 place of water immersion high powers in Histologi- 

 cal research, as they have no correction for thickness of 

 cover glass, and are consequently much easier to use, 

 the only drawback is that the essential oil used will 

 dissolve Canada balsam, Dammar varnish, and many of 

 the other sealing fluids, and it is necessary to cover them 

 with Hollis' glue which is not acted on by cedar oil. 



These glasses were first made by Zeiss of Jena, and 

 since by Messrs. Powell and Lealand. The first glass 

 made by Zeiss was the J-, he then brought out a T ^- 

 and afterwards a J^, of these the T ^ is unquestionably 

 the best glass. Messrs. Powell and Lealand have made 

 _3^ ? _!_, _i_ y and -^3- on this principle, and have succeeded 

 in removing a great objection to oil immersion lenses ; 



