The Influence of Cover-glass on the Tube-length 69 



Gradually lengthen the tube, or turn the collar, repeating 

 the above observation after each change, until all evidence 

 of spherical aberration has disappeared ; the instrument is 

 then in correct adjustment within your own limits of vision. 



It is advisable to start with the adjustment corresponding 

 to the thickest cover, for the simple reason that this lessens 

 the danger of running through the cover-glass and destroying 

 the object, and possibly the front lens of the objective, when 

 dealing with a lens of a short working distance. 



The difference between an objective adapted to a 6-inch 

 and that for a 10-inch tube is, that in the latter case the 

 back combinations of the objective are brought closer to 

 the front lenses ; this gives a slightly increased aperture. 

 The majority of cover-glasses that are purchased and a large 

 number of those used over commercial objects are more than 

 0*008 inch thick ; 0*008 inch is a medium thickness of cover- 

 glass, but the tendency is to use thicker ones. It will be 

 found a great advantage to buy only such objectives as are 

 corrected for the 10-inch tube, and having the rack-work 

 before referred to fitted to the microscope tube, sufficient 

 latitude would still be allowed if a thinner cover-glass were 

 met with ; but it would often be found necessary to close 

 the draw-tubes down to 6 or 7 inches, in order to get the best 

 correction for the thick cover-glasses that are commonly 

 used. 



We may here clear up another question that occasion- 

 ally arises. If a -J--inch objective is corrected for a 

 6-inch tube-length, it does not give a magnification of 60 

 diameters at 6 inches. The powers of all objectives are 

 calculated for a 10-inch tube-length, therefore the full total 

 benefit is not obtained from an objective when used at 

 6 inches, but only six-tenths of it. Of course, with the 

 lessened magnification at 6 inches a brighter field is pro- 

 duced, and a deeper power of eyepiece is found permissible. 

 This is rather an important item in testing an objective, 

 because an objective at 10 inches would be yielding about 

 two-thirds more magnification than at 6 inches, and its 



