USE AND MANIPULATION OF MICKOSCOPE 157 



distance so found would require to be multiplied by the 

 refractive index of the glass (approximately f ), and this would 

 give the approximate thickness of the cover. It is, however, 

 clearly of the nature of a makeshift method, since no allowance 

 is made for possible variations in the thickness of different parts 

 of the same cover-glass a factor by no means negligible or for 

 the cases in which the part of the object in view lies a greater or 

 less distance below the cover-glass. It is not nearly so exact or 

 so satisfactory in any way as direct observation of the effect 

 that collar-adjustment or alteration of tube-length has on the 

 image itself. 



Eeference to Fig. 56 a and b will enable a better idea to be 



FIG. 56. Influence of Cover-glass Thickness on Adjustment. 



obtained of the manner in which this is to be effected. Fig. 56 a 

 shows the path taken by rays of light which come from a 

 perfectly corrected lens system, and which form an image so 

 that all the rays pass through one point. Fig. 56 b shows the 

 image formed by a lens system that is not in correct adjust- 

 ment, and in which the rays do not come to one point. It is 

 evident from Fig. 56 a that if the rays are examined at the 

 position A or B, instead of at the point F, although the object 

 will be out of focus the images given will be identical whether 

 we examine them on the one side or the other of the correct 

 focus. In either case a disc of light will be seen, and these 

 discs will be equal in brilliancy ; the disc A, in fact, will be 

 exactly the same in every particular as the disc B. Such, 

 however, is not the case with Fig. 56 b. The image in the 

 position A will here be very much more brilliant than in the 



