CH. IX] 



TUBE-LENGTH OF THE' MICROSCOPE 



287 



effect is the same as thinning the cover-glass. That is, it introduces 

 under-correction. This makes it possible to compensate for too 

 thick a cover by shortening the tube ( 135, 462). 



If the tube is made longer than the standard it has the same effect 

 as using too thick a cover-glass. It therefore introduces over-correc- 

 tion, and if a cover too thin has been used it may be compensated 

 for by lengthening the tube. 



When homogeneous immersion liquid is used one does not have to 

 trouble about the exact thickness, but care must be taken not to 

 use so thick a cover that the working 

 distance will be too short ( 76). 



By consulting the catalogues of micro- 

 scope manufacturers one can find for 

 what tube-length and thickness of cover- 

 glass their unadjustable objectives are 

 corrected. For example, in the 1914 

 editions oi the catalogues of the Bausch 

 & Lomb Optical Company of Rochester, 

 and of the Spencer Lens Company of 

 Buffalo, it is stated that the tube-length 

 is 1 60 millimeters and as shown in the 

 accompanying figure (fig. 170), it in- 

 cludes the parts from the upper end of 

 the draw-tube to the nut into which the 

 objective is screwed. 



The cover-glass thickness is given as 

 o.i 8 millimeter, and the user is warned 



that for the higher powers a variation in thickness from this standard 

 of 0.03 or 0.04 mm. would deteriorate markedly the perfection of the 

 image. The statement is furthermore made that with the homo- 

 geneous immersions no harm would result, but on the other hand 

 great care must be exercised there to use the correct tube-length or 

 aberrations will be introduced. 



CONDENSER 



FIG. 170. THE MICROSCOPE 

 SHOWING TUBE-LENGTH. 



