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In the following papers I hope to have the honour of 

 describing some results of experiments carried forward for 

 many years, which have for their chief object the increase of 

 power and the advancing correction of residuary aberration, 

 combining, also, a deeper penetration imparted to micro- 

 scopic objectives, and greater distance between the delicate 

 object lens and the object observed. In the following objec- 

 tives of focal lengths — 



Avere measured intervals between the front lens, or rather its 

 setting, and the covering glass ; and in proportion as the 

 tube is lengthened in general, this small space diminishes as 

 the power so gained increases, and except the covering-glass 

 is extremely thin with the higher powers, the objective dan- 

 gerously comes down with pressure upon it in the act of 

 attaining distinct vision. 



" Penetration," or the actual depth to which distinct vision 

 reaches through a transparent object, is one of the most essen- 

 tial conditions for correct observation, and the extreme shal- 

 lowness of the highest powers is one of the most formidable 

 difficulties encountered in the interpretation of microscopical 

 phenomena. The gorgeous brilliance, depth, and contour of 

 illuminated objects (as the heads of insects) under a 3-inch 

 objective form a striking contrast with the effect of an eighth, 

 which presents to the eye sections formed by the focal planes 

 of vision. 



As power is increased, of course the front glasses approach 

 the object until with the 1 — 16tli the separation diminishes to 

 three or four thousandths of an inch. Difficulties of observa- 

 tion now increase. A great variety of interesting objects 

 already collected cannot be seen at all. Opaque objects are 

 out of the question. With such a power the front lens is 

 generally in contact, or nearly so, with the protecting cover- 

 ing glass. It is almost impossible to insinuate an illumi- 

 nating ray between it and the object. 



But even with the best arrangements difficulties of defini- 

 tion in individuals resemble the personal errors of astrono- 

 mers eminently depending upon personal idiosyncracy. The 

 actual powers of vision in the abstract are still debated. 

 Certain persons within my own acquaintance have distin- 

 guished objects of a size and at a distance quite incredible — 

 as the positions of Jupiter's satellites and the time by a 

 distant clock. 



In the microscope, to an eye adjusted to ten inches disiinct 



