4 • The Microscope. 



Although I am aware that many eminent microscopists do 

 not favor adjustable objectives for every day work, I must con- 

 fess that I fail to see the force of their arguments. From the 

 foregoing it will be seen that unless the cover-glasses are of a 

 thickness corresponding to that which was originally used, the 

 objective may be made to do imperfectly what is in its 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 manipulator'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- 

 adjustable 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 ob- 

 jectives ? 



Another factor in the disturbing influences is the variation 

 in length of tube; the deleterious results are similar to those 

 with varying cover-glasses. Objectives 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 object- 

 ives and eye-pieces. That it is so in objectives is patent, and 

 that it is so in eyepieces 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 spherically under-corrected and vice versa when it 

 is increased, so that it is apparent that by the use of the draw- 

 tube the effect of the cover-glass may be partially neutralized ; 

 for instance when by the use of a thi'n cover, the objective is 

 sperically under-corrected, it may to a certain extent be cor- 

 rected by causing a corresponding over-correction in the tube 

 by increasing its length. The use of the draw tube for the pur- 

 pose of changing the amplification or for the matter of conven- 

 ience, can therefore hardly be commended, except in cases 

 where adjustable objectives are used. 



