The Influence of Cover-glass on the Tube-length 67 



is a matter of extreme delicacy, which can only be accurately 

 done as the result of experience and with the aid of a critical 

 eye. It is hardly to be recommended to students, because 

 they will not usually afford the time and trouble necessary 

 to get such perfect results ; consequently there is a growing 

 tendency, except in the apochromatic objectives, to have 

 the lenses mounted in a rigid setting, corrected for a specific 

 tube-length and thickness of cover-glass. With the fixed 

 setting, if a different thickness of cover-glass be used than 

 that for which the objective was designed, correction can be 

 made by altering the tube-length of the microscope. This 

 has the same effect as altering the distance between the 

 lenses. Supposing we had an objective adjusted for a 

 6-inch tube, with a ' B ' eyepiece, on a cover-glass O'OOS 

 inch thick (this is about the average thickness adopted by 

 opticians), and we wished to examine an object having 

 another thickness of cover — say 0"005 — we should at once 

 notice that the performance was not so good, and in order 

 to improve it we should have to make the body longer. 

 This difference of cover-glass thickness, with a good ^-inch 

 objective, would necessitate a 10-inch body. On the other 

 hand, if the cover were 0"01 inch thick, the length of the 

 body would have to be diminished below that for which the 

 objective had been designed, to obtain the best results — 

 that is, for a thin cover the body-tube would have to be 

 lengthened, while for a thick one it would have to be 

 shortened, and the finer the quality of the objective the 

 more sensitive would it be to cover-glass thickness. This 

 system of correcting by draw-tube, however, has one draw- 

 back, and that is, that the power is varied in correcting, 

 and, of course, the focus is altered. Prom the considera- 

 tions here named, it will be found advantageous if the 

 microscope be provided with a means of lengthening the 

 body by draw-tubes to 12 inches, and on the other hand, 

 when the draw-tubes are closed, of having the body shorter 

 than the Continental length (6 inches). In order that the 

 best adjustment maybe made, it is essential that one of the 



5—2 



