1885-86.] Editiburgh Naturalists' Field Club. 327 



achromatism, and for long after, made little progress. Who has 

 not seen a compound non-achromatic microscope as constructed 

 about the beginning of this century, with its huge tube and wheel 

 of small lenses forming a multiple " nose-piece," upon turning 

 which each lens in succession gave a different power ? Any one 

 who has examined an object with one of these glasses, and con- 

 trasted it with a modern objective, must be astonished that they 

 could even be tolerated, and yet they are bought and sold at the 

 present day. The invention and perfection of the achromatic 

 microscope objective has advanced with such leaps and bounds, 

 that it may well rival the steam-engine, with the rapid improvement 

 of which it has been almost a contemporary. The principle of 

 achromatism was applied successfully to the telescope just about 

 100 years ago. It was not, however, until the 5th April 1824 

 that M. Chevalier successfully made an achromatic microscope 

 objective, which was exhibited to the French Academy, This was, 

 no doubt, the date of the birth of the present compound achromatic 

 microscope, and worthy of remembrance. About the same time 

 Mr TuUey, in England, made one for Dr Goring of nine-tenths of 

 an inch in focal length. 



There are two, and only two, special difficulties to be overcome 

 in the construction of objectives. I cannot give here any explana- 

 tion of them, but merely name them — (1) Spherical aberration ; 

 (2) Chromatic aberration. Both these are fully explained in the 

 text-books, but they are of such a nature that the very highest 

 skill and talent are necessary to overcome them to an extent neces- 

 sary to make an objective. Neither in the one case nor the other, 

 however, can they be, or are they ever fully eliminated. An objec- 

 tive is said to be aplanatic, or have a flat field, when the spherical 

 aberration is destroyed to a certain extent ; and it is said to be 

 achromatic when the image it gives is free from colour. Now when 

 objectives were first made, it was found by placing two or three 

 lenses, all of which were achromatic, above one another, that the 

 spherical aberration was cured or destroyed to a large extent, 

 and this system has been followed in their construction. The 

 French make a large number of lenses all of like foci, and by a 

 method of selection of the best, or those best matched to each, 

 they construct a tolerably good objective. This is said to be 

 "marrying" them. Objectives of this kind, however, have in- 

 variably a small angle, admit little light, and are not adapted 

 for the finest definition constantly required by the microscopist. 

 It is somewhat strange that although the initiatory steps in the 

 perfecting of the microscope are nearly all due to foreigners, it is 

 to our own countrymen that the real advances are mostly owing. 

 After it was once seen to be possible to construct achromatic 

 micro object-glasses, improvements went on very rapidly. The 



