116 



power to do well, and when pressed to its full capacity 

 may and is likely to fail. It must be remembered 

 that cover-glasses of the same number are not of the 

 same thickness. The selection of those of proper 

 thickness is expensive and tedious, whereas the 

 knowledge of correcting the objective is easily 

 acquired, and in the latter case it is in the manipu- 

 lator's power to command the highest performance of 

 which the objective is capable ; further than this, it 

 has the advantage that it may be used as a non-adjust- 

 able objective if desired. When homogeneous immer- 

 sion objectives were first introduced they were mounted 

 in fixed settings, as it was expected that the thickness 

 of the cover-glass would not affect the correction ; 

 although this assumption was correct, it was found 

 that even in these it was necessary. How much more 

 then, is it required in dry or water immersion objec- 

 tives ? 



Another factor in the disturbing influences is the 

 variation in length of tube ; the deleterious results 

 are similar to those with varying cover-glasses. Objec- 

 tives are usually adjusted to 8| or 9 inches length of 

 tube, and although this in itself is a fixed standard, it 

 usually becomes variable by changing objectives and 

 eye-pieces. That this is so in objectives is patent, 

 and that it is so in eye-pieces can easily be determined 

 by making a change in powers, when it will be found 

 that a change in focus is required. By decreasing the 

 length of tube the objective will appear to be spheri- 

 cally under-corrected and vice versa when it is increased, 



