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



A is the additional lens (concave in I. for a thick cover, convex 

 in II. for a thin) ; 



U and L are the upper and lower limits, respectively, of the 

 draw-tube ; the image must be formed between these limits to 

 be capable of being focused by the eyepiece ; 



Vis the point at which the objective must produce the image, 

 if it is to be the best possible ; it will be seen that in each case V 

 lies outside the limits of the draw- tube, so that the best possible 

 image could not be focused by the eyepiece ; 



T is the point, well within the limits of the draw-tube, to which 

 the image V is transferred by the additional lens. 



The actual path of the rays is in each figure shown by dark lines ; 

 the broken lines show the paths that would be followed in the 

 absence of the additional lens. 



It would, no doubt, be possible, by the use of the focusing 

 adjustments of the microscope, to bring an image of a sort, in 

 either case, within the limits U, L ; but it would not be the best- 

 possible, indeed in the majority of cases it would be very inferior, 

 owing to the tube-length being incorrect ; the function of the 

 additional lens is to allow the objective to work at the proper 

 tube-length OV, but to bring the " best possible " image, formed 

 at the proper tube-length, within the available limits of the draw- 

 tube. 



With objectives of not too high power the ordinary l/6th- 

 in., for example there is scarcely any limit to the amount 

 of correction which can be produced in this way. Take, 

 for instance, an objective (Watson 4 mm. Apochromat, 

 N.A. 0'85) corrected in the usual way to work through a 

 cover-glass 0'18 mm. in thickness, working on an uncovered 

 object, and it will be seen that the definition is good. In 

 this case a convex lens of + 4 diopters is placed behind the 

 objective. W T ith a dry l/8th-in., however, or with dry ob- 

 jectives of still higher power, it is not possible to go quite 

 so far as this, though the results to be obtained are by no 

 means bad. 



I am showing the objective working on an uncovered object 

 with a view of demonstrating the amount of correction that 

 may be obtained in this way ; at the same time I ought to 

 say that this is not the best way of making an ordinary objective 

 work on an uncovered object : the best and easiest method is by 



