146 HOW TO SEE WITH THE MICROSCOPE. 



their aperture to any thing like an approach to 180. 

 Among such may be included the two inch of 25, the 

 one and one-half inch of 37, the one inch of 40, the 

 two-thirds of 45, the one-half inch of 100, the four- 

 tenths of 150, and so on. All the glasses above 

 named may be considered as high-angled objectives. 

 They should be nicely corrected, fully up to the limits 

 of the aperture claimed for them, and, as compared 

 with similar lenses of narrow apertures, they should 

 possess a much greater power of light, and bear higher 

 eye-piecing; and, let it be borne in mind, that of the 

 above-named objectives, the one having the greater dis- 

 tance will be "endowed" with the greater " penetra- 

 tion." 



And right at this place let us see what the " power 

 to bear high eye-piecing" means. To illustrate this, 

 we will take a case such as might happen in practice. 

 For example, I desire to see the transverse lines of 

 pleurosigma balticum, and, as mounted on the Moller 

 balsam test plate, their striations being from 31 to 34, 

 in .001 Eng. inch. Now, I attack this with, say, a 

 real good low-angled one-half inch, and find that I can 

 just get a glimpse of the lines with the one-half inch 

 eye-piece. I then apply the one-quarter inch eye-piece, 

 and find that the view is not nearly as satisfactory as it 

 was with the lower ocular; hence I conclude that the 

 one-half inch eye-piece is as high as the objective will 

 bear. Now, removing the one-half inch objective, we 

 will substitute a two-thirds, but of higher aperture. 

 With the latter glass, using the one-half inch ocular, I 



