561 



AN ADDITION TO THE OBJECTIVE. 



By M. A. Aixslie, R.X., B.A., F.R.A.S. 



{Read April 21th, 1915.) 

 Figs. 1 and 2. 



Probably there are few microscopists who are in the habit of 

 using high-power dry objectives who would not agree that the 

 Correct adjustment of the tube-length to suit the thickness of 

 the cover-glass is with such lenses of great importance if good 

 definition is to be obtained. Definition of a sort may, it is true, 

 be got with incorrect tube-length ; but only by unduly closing- 

 down the iris diaphragm and thus reducing the illuminating 

 cone, or otherwise interfering with the uniformly illuminated back 

 lens which is the basis and starting-point of all correct microscopic 

 vision. In the present paper I am not considering inferior 

 definition got in this way ; I am only considering the question 

 of obtaining really sharp definition, with a cone of illumination 

 which utilises at least two-thirds of the aperture of the objective ; 

 and this is what I mean when I speak of " good ' ' definition ; 

 and such is only to be obtained by careful adjustment of the 

 tube-length to suit the thickness of the cover-glass. 



With objectives fitted with correction collars this paper has 

 not much to do ; the correction collar to a large extent obviates 

 the change of tube-length without interfering much with the 

 magnifying power, and is useful in other ways, as for example 

 in focusing through the various planes of a thick object ; but 

 unfortunately it seems to be going out of use, except in the dry 

 apochromats and in water-immersions ; and as 99 out of 100 of 

 the high-power dry objectives met with at the present time 

 are without this appliance, I shall not take it into further con- 

 sideration, but confine my attention to the objective as com- 

 monly used. 



If we open a treatise on Microscopy, we are pretty sure to 

 find the question of cover-glass and tube-length alluded to more 

 or less (usually less) fully. The reader is told that for a thick 



Jourx. Q. M. C, Series II. No. 77. 



