Ohjeci-glasses and their Definition. 221 



has to be thrown out of adjustment for the sake of accommodating 

 errors from superadded lenses, the investigation does not hold out 

 much promise, and I regret that in no instance have I been able to 

 define any description of error in object-glasses that may lead to their 

 practical improvement, by the aid of such intermediate lenses, as these 

 derange the very corrections that the optician has been striving to 

 make perfect. Enormous magnifying power may be gained by such 

 lenses, but in effect this is far inferior to that obtained directly by 

 high power objectives, and may give rise to illusory appearances. 

 The inquiiy is by no means exhausted, and something may yet 

 come of it in the shape of an universal microscope, as an improve- 

 ment upon that manufactured by Smith and Beck. This was almost 

 exclusively used with a f rds object-glass, rather differently corrected 

 with an oblique-cut adjusting collar which gave a considerable range 

 (near ith of an inch, if I recollect right) between the lenses, which 

 were to be separated in degree as the draw-tube was extended. 



In a recent trial I have found that with a -Yo^hs, or ith, an erect- 

 ing glass or intermediate combination consisting of two achromatics of 

 1 inch or 1-i^inch focus with an interval of about 1-Hnch between 

 them has given the best result. The shorter focus or one nearest 

 the object-glass should be considerably under-corrected, and the other 

 over-corrected to a counteracting degree. By altering the distances 

 between them, the second image formed by the arrangement can be 

 improved for the aberrations, which vary for every position that 

 they occupy in the body of the microscope. 



The foregoing was written previous to the appearance of the 

 last number. In reference to actinic glasses, I may state that when 

 I practised micro-photography some eighteen years ago, I found it 

 necessary to use object-glasses in which the visual and chemical foci 

 were coincident in one plane : this I accomplished by adding another 

 single lens set close behind the others, of a focus suitable for obtain- 

 ing the result. Colonel Dr. Woodward in using the dry lens, 

 replaced one of the combinations by another, giving an identical 

 visual and actinic focus. He states in his recent paper " that the 

 immersion objectives of high power requu'e no special corrections to 

 fit them for photographic use, and are the very ones actually 

 employed." This is a valuable property of the immersion lens that 

 has not before been noticed and is correct in theory, for the adjust- 

 able thickness in the front, which the water film permits, is self- 

 compensating, and will at once produce the required under-correction 

 for coincident foci, so that no photographic Hnes are separate from 

 the visual ones, and Dr. Pigott's authority that errors may arise 

 from this cause cannot be accepted. 



