564 M. A. AINSLIE ON AN ADDITION TO THE OBJECTIVE. 



I now come to the device which I am bringing to your notice 

 for overcoming the difficulty caused by insufficient range of 

 draw-tube. 



Many years ago the late Dr. Van Heurck used what he 

 called a " transformer," for the purpose of enabling short- 

 tube objectives to work on the long tube, and vice versa. I 

 do not know that he used it for any other purpose, or with 

 a view of compensating for insufficient range in the draw- 

 tube. 



He applied, behind the objective, a lens of small power, either 

 convex or concave, according to the effect desired, and stated 

 that in this way he was able to use even a 2-mm. Apochromat, 

 corrected for the short tube, on the long tube, without any 

 appreciable loss of definition. The lenses he used were, I believe, 

 achromatic. 



But it has occurred to me that the utility of this device is of 

 far wider range than this. In the course of a series of experi- 

 ments with a large number of dry objectives of various (high) 

 powers, I have found that it is possible to increase very greatly 

 the range of thickness of cover-glass through which the objective 

 will give good definition ; and I hope, later on, to show another 

 use for this additional lens, which has not, so far as I am aware, 

 been described before. 



If a convex or concave lens of low power be introduced im- 

 mediately behind the objective, it has the effect of altering the 

 degree of convergence of the rays of light projected by the back 

 lens of the objective, and thus of altering the position in which 

 the image is formed. Conversely, if the objective requires, to 

 give good definition, that the image should be formed in a plane 

 either within, or beyond, the available limits of the tube, it is 

 perfectly possible, in the great majority of cases, to find a lens 

 of such a power that its introduction above the objective will 

 bring the image within the limits of the tube. 



Suppose, for example, that we have a cover-glass so thick that 

 the objective will only form a perfect image of the object at a 

 point too close to the back lens for the tube to be sufficiently 

 shortened ; it is true that we can, by using the focusing adjust- 

 ments, bring the image to the top of the tube, but then we do 

 not get a perfect image ; that is to say, not perfect in the sense 



