Objectives. 45 



turer. It should be borne in mind that the ordinary 

 Wenham binocular will not work with an objective having 

 a larger air angle than 40. Most objectives having a 

 numerical aperture of more than 0'66 have what is termed 

 a correction collar, whereby the two back combinations of 

 the objectives are removed farther from, or brought closer 

 to, the front lens. As a rule, objectives are corrected for a 

 certain thickness of cover-glass, which is placed over the 

 object to protect it ; but these cover-glasses vary consider- 

 ably, and consequently disturb the corrections of the objec- 

 tives. In order to compensate for this, collar adjust- 

 ment is added. Its method of use is as follows : Set the 

 lens at zero in the direction marked, uncovered, on the 

 objective, focus some dust on the surface of the cover-glass, 

 then turn the adjustment collar until the object itself 

 beneath the cover-glass comes into view ; the objective will 

 then be corrected for that thickness of cover-glass. Ex- 

 perience alone can show how to do this correctly and to the 

 best advantage, but it can hardly be recommended to 

 students, as they cannot afford the time and trouble 

 necessary to get such perfect results, and prefer the fixed 

 mount. Corrections may be made for the fixed mount by 

 shortening the body tube of the microscope should the 

 cover-glass be thicker than that for which the objective 

 is intended, and lengthening it if it be thinner. 



Since the introduction of the new glass great improve- 

 ments have been made in the cheaper class of achromatic 

 lenses, many of them almost rivalling the apochromatics ; 

 but these are individual glasses, from several makers' 

 series, the No. 6 leus by Keichert being a specially fine 

 specimen. They are extremely sensitive to variation of 

 thickness of cover-glass, and it will be found advantageous 

 if the instrument be provided with the means of lengthen- 

 ing the body by drawtubes to 12 inches ; and, on the 

 other hand, when the drawtubes are closed, of having a 

 body shorter than the Continental length (6 inches). In 

 order that the best adjustment may be made, it is very 



