THE ONE-INCH OBJECTIVE. 



IS 



the corrections required are easily made on this system. 

 The triplet, used only for low angles, is the least to be 

 commended, and should not be used with deep eye-pieces. 

 The i -inch objective may be a triplet, 

 as shown in Fig. 28, or a double combi- 

 nation, as in Fig. 29. 



In the latter the front is a plano-con- 

 vex of crown, with a meniscus of flint, 

 being separated by a considerable inter- 

 val from the posterior combination, which 

 is composed of two double convex crown 

 lenses, holding between them a double 

 concave of flint. 



It will be seen from Figs. 28 and 29 



FIG. 28. 



that there is more work in the construction of the higher 

 angle. The cost is consequently greater ; but when we 

 remember that the i-inch of 25 admits more light than the 

 one of 1 6, that it defines better, resolves 

 better, and proves to be a much superior 

 working glass in every respect, the 

 extra money will not perhaps be grudged 

 for it. 



Half-inch objectives are made on two 

 systems : the low angles for binocular 

 use of a thick solid front, at the back 

 of which are two pairs of partly cor- 

 rected lenses, the aberrations being 

 finally corrected by the thickness of the 

 front. The higher angles are constructed 

 of three pairs of lenses, the posterior 

 combination being of considerable width. They are made 

 of as high an aperture as o'66 or 82 air angle, while the 

 A-ths of 100 air angle, or numerical aperture 0-76, is far 

 from uncommon. 



E 2 



FIG. 29. 



