AND ON THE ABSORPTION OF THE PRISMATIC COLOURS. 437 



torn of a concave dish of platinum, kept very hot by a spirit- 

 lamp placed beneath it. The bottom of the dish is made with 

 a number of projecting eminences, in order that the film of fluid 

 which rests upon it may be exposed at many points to the action 

 of the heated surface. See Fig. 2. After the lamp has burned 

 for some time, a portion of unevaporated water, mixed with a 

 small quantity of alcohol, will remain at the bottom of the 

 dish, in a state unfit for combustion. This water may be taken 

 up by a sponge, or it might be prevented from accumulating, 

 by having a fountain of pure alcohol, from which the exhaust- 

 ed strength of the diluted fluid could be renewed. 



. The Monochromatic Lamp being thus completed, I lost no 

 time in applying it to the illumination of Microscopic objects. 

 The effect which it produced far exceeded my expectations. 

 The images of the most minute vegetable structures were pre- 

 cise and distinct, and the vision in every respect more perfect 

 than it could have been, had all the lenses of the microscope 

 been made completely achromatic by the most skilful artist. 

 The errors which arise from the different refrangibility of 

 light, being removed from microscopical observations, I was 

 enabled to enlarge considerably the aperture of the object- 

 glass, till the image became sensibly affected by the errors of 

 spherical aberration. In order to diminish these as much as pos- 

 sible, the object-glass should consist of an annular portion of the 

 lens that gives a minimum spherical aberration, and while its 

 surfaces are nicely centered, their common axis should be ad- 

 justed so as to coincide accurately with that of the other lenses. 

 The optician, however, will probably be induced to exert his 

 ingenuity in the formation of elliptical and hyperbolical lenses, 

 which, since the days of Descartes and Huygens, have exist- 

 ed only in optical theories, and thus remove the only imper- 

 fection which still attaches to the microscope. 



Independent 



