562 M. A. AINSLIE ON AX ADDITION TO THE OBJECTIVE. 



cover-glass the tube is to be shortened, and that it is to be 

 lengthened for a thin ; and the importance of the matter is 

 impressed on the reader, even to the extent of saying that " the 

 correction of the objective and the tube-length ought to vary 

 with every object" (Dallinger), a statement which would appear 

 to require some modification in the case of oil -immersion objec- 

 tives. 



Little, however, is usually said as to how the correct tube- 

 length is to be recognised when obtained, and nothing as to 

 how much we may expect to have to move the draw-tube. On 

 this latter point I hope to give some data which may prove 

 useful. 



One of the first things to strike any one who tries to examine 

 a few mounted specimens, with, say an l/8th, is that the range 

 of draw-tube of the modern stand is often insufficient to allow 

 for more than a very slight variation in the thickness of the 

 cover-glass ; and this is more particularly the case with stands 

 of Continental make, in which the available range is often not 

 more than 50 mm. ; and to give some idea of what a hindrance 

 to observation this may prove, I may say that with an English 

 stand (by Watson) having the good range of tube-length of 92 mm. , 

 -and using a Leitz No. 7 (which is a 1/8 tli of N.A. 0*85), I have 

 found it impossible to examine some of the beautiful slides of 

 Diatomaceae in the Club Cabinet, in some cases because the 

 cover-glasses were too thin, and in other cases because they 

 were too thick. With the limited range of the Continental 

 draw-tube one would, of course, be still worse off. 



Another point on which the text-books are silent, but which 

 soon becomes evident to any one who has occasion to use objec- 

 tives of different powers, is that the change of tube-length 

 necessary to correct for a given variation in the thickness of 

 the cover-glass is not always the same ; it varies enormously 

 with the power of the objective, and also, to some extent, with 

 the formula on which the objective is constructed. At one end 

 of the scale we have such objectives as the half-inch " Holos ' 

 of Watson & Son, N.A. 0*65, which requires a change in the 

 tube-length of about 1*2 mm. only to compensate for a variation 

 of 0*01 mm. in the thickness of the cover-glass ; and the Zeiss 

 12 mm. Apochromat, of the same N.A., which requires a change 

 of about 2 mm. under the same conditions. 



