114 Transactions of the Society. 



lined objects can be viewed, or lined appearances produced, has 

 been to throw fresh doubt as to the correspondence between actual 

 structure, and optical effects of this description ; and we are thrown 

 back, as I pointed out in reference to insect scales long ago, upon 

 the necessity of making various experiments, and of reasoning 

 from the best analogies we can obtain in the interpretation of the 

 appearances we see. Amongst those who have experimented with 

 the oil lenses I may mention Dr. Pigott, who speaks of a No. 15 

 as a " magnificent glass." 



It was not found by Mr. Dallinger, whose researches require 

 the highest and most efficient optical aid, that the oil objectives 

 showed anything he could not see with the Powell and Lealand 

 glasses he usually employed, but he at once admitted and praised 

 the great facility with which they could be used. He has since 

 obtained excellent effects by using oil of cedar wood with Powell 

 and Lealand's newest I, yV» ^^^1 yV? and expresses great admiration 

 for a new wet and dry i^V by the same makers. 



By the kindness of Mr. Baker, I had an opportunity of trying an 

 \ and tV- The latter I found a splendid glass, remarkable for the 

 ease with which it displayed difficult objects, and requiring only a 

 film of oil to connect it with the slide cover. The a had such a 

 large working distance that it required a little puddle of the oil, 

 and on this account could only be employed with the Microscope in 

 a vertical position. I am told that these glasses ^e not all alike in 

 this respect. ^' •<, 



The ' American Quarterly Microscopical Journal ' for January, 

 1879, contains a letter from Professor Abbe, explaining that owing 

 to a mistake several |ths have been made with a balsam angle of 

 107° to 109^ only, instead of 114 to 116, as they should have 

 been. He also states these objectives are composed of " four sepa- 

 rate lenses," and not three, as Professor H, L. Smith supposed. 



It seems probable, from Mr. Dallinger's experiments, and a few 

 made by myself, that opticians may be able to furnish us with 

 objectives that can be corrected for either oil or water ; but if this 

 cannot be satisfactorily accomplished, it is not likely that the oil 

 lenses will supersede the water ones, though they have obviously 

 some decided advantages for special purposes. 



The fine performances of the large-angled glasses of the best 

 makers, and the results obtained with the extreme angles of the oil 

 lenses, have led in some quarters to a belief that great angles are 

 good for all purposes ; but there can be little doubt that this will be 

 found a mistake, and that glasses of small and moderate angles, 

 with fine corrections, will still be needed for much, and probably 

 for most, valuable work.* 



* See Dr, Pigott's paper " On the Invisibility of Minute Refracting Bodies 

 caused by Excess of Aperture," * M.M. J.,' February, 1875, p. 55. 



