20 



Aehromatie Objectives. 



Aided by the considerably extended list of materials produced by the local 

 Glass Works we have reconstructed most of the older types of our achromatic 

 objectives, the higher powers of the dry system have, moreover, received single 

 lenses of fluorite, so as to obviate in these objectives also, more completely than 

 had formerly been possible, imperfections due to spherical and chro- 

 matic aberration. With those systems of the dry series which have a numerical 

 aperture exceeding 0.65 this improvement, apart from greater sharpness and 

 clearness of the image, is also manifested by increased sensibility to 

 differences in the thickness of the cover-glass and the length of 

 tube. Consequently, in the use of these objectives now even closer attention must 

 be given to these two points than formerly. 



6. Objectives at, si2, 113, 



attached to the tube. 



(V, Full Size.) 



Fig. 7. Objective a*. 



By rotation of the ring RR the 



upper pair of lenses (i 2 ) may be 



elevated into the position L,' , 



indicated by dotted lines. 



(Full Size.) 



Our lowest power objectives, viz: a . at, 82 and 83, are simple achromatic 

 lenses, so mounted, that, notwithstanding their great focal length, the body of 

 the microscope remains at its ordinary elevation during observation. For this 

 purpose lenses a and at have their threads so situated that, when screwed into the 

 tube, the lens is inside the body (see at, Fig. 6), and they cannot therefore 

 be used with a revolving nose-piece or other objective-changer. 

 Though the former is primarily intended for use with our binocular microscopes,- 



