M. A. AIXSLIE ON AX ADDITION TO THE OBJECTIVE. O/O 



glass " point of view, perhaps the most remarkable use to which 

 the additional lens can be put. 



This is the conversion of a dry objective into an oil-immersion. 

 In the great majority of cases this cannot be done, not only 

 because it is too radical a change for most objectives, but because 

 the working distance is as a rule too great to admit of oil-contact. 

 But there are certain objectives of comparatively low power, 

 and small working distance, with which it is possible. The 

 matter only occurred to me a day or two ago, so that I have not 

 had the opportunity of trying the experiment with more than two 

 lenses ; with one of these, however, a 6-mm. Holos of the earlier 

 construction, having an N.A. of 0*84, I succeeded fairly well. 



The additional lens required is a concave of 10 diopters ; 

 with this, though the field is much curved, and good definition 

 can only be obtained in the centre, the effect is quite good. 



It will be realised that the substitution of oil for air between 

 the front lens and the cover-glass is optically equivalent to the 

 thickening of the cover-glass so as to fill the w T hole space between 

 the object and the front lens : it might be expected, therefore, 

 after w T hat has been said, that the power of the additional lens 

 required would be considerable ; and I doubt whether the effect 

 could be obtained with a l/6th-in., except perhaps with one 

 of very short working distance. 



The advent of the Zeiss l/7th-in. oil-immersion drew atten- 

 tion to the better resolution given by an oil-immersion over that 

 given by a dry objective of the same aperture ; but I did not 

 expect that this would hold good with such an arrangement as 

 that here described. I was much surprised, therefore, to find 

 that it was possible to resolve A. Lindheimeri with a solid axial 

 cone of illumination, the longitudinal and transverse striae being 

 quite plain with a compensating eyepiece 16*5. In this specimen 

 the striae run about 70,000 to the inch, and it is a very severe 

 test, under the conditions of illumination mentioned, for any 

 dry lens ; it is, of course, too hard for the 6 mm. Holos in its 

 dry state, the aperture being insufficient. The introduction 

 of the convex lens increases the N.A. to about 0'89 ; but I think 

 it is safe to say that no dry lens of this aperture would effect 

 the resolution with central light, though a much smaller aperture 

 will suffice with oblique light. In this case, then, the conversion 

 to an oil-immersion affords a distinct gain. 



