226 



MICROSCOPE. 



v impound ved by Dr. Wollaston, as described in p. 218. We can- 

 Mioros- n ot, therefore, expect any essential improvement in the 

 topes. s i n gle microscope, unless from the discovery of some 

 r ~^~ m transparent substance, which, like the diamond, com- 

 bines a high refractive power, with a low power of 

 dispersion. 



" In the combination of single lenses to form the com- 

 pound microscope, opticians have likewise arrived at a 

 great degree of perfection. The aberration of refran- 

 gibility can now be completely removed by a suitable 

 arrangement of the individual lenses; and every arti- 

 fice has been exhausted in suiting the apparatus to the 

 various tastes of purchasers, and to every purpose of 

 popular observation. 



" No attempt, however, appears to have been made by 

 opticians to fit up the microscope as an instrument of 

 discovery, to second the labours of the naturalist in 

 preparing the subjects of his research, and to accom- 

 modate the instrument to that particular kind of prepara- 

 tion which is indispensibly necessary for the preservation 

 and inspection of minute objects. 



" In perusing the writings of those naturalists who 

 have applied the microscope to the examination of mi- 

 nute objects, we find, that the most difficult and per- 

 plexing part of their labour consisted in preserving and 

 preparing the different insects and substances which 

 they wished to inspect. Small insects instantly shrivel 

 up and lose their natural form as soon as they are kill- 

 ed, and the minute pnrts of plants suffer a similar 

 change from exposure to the air. Hence Swammerdam 

 and Lyonet killed the insects which they meant to ex- 

 amine, by suffocating them either in water, spirit of 

 turpentine, or diluted spirits of wine. The softness 

 and transparency of their parts were thus preserved du- 

 ring the process of dissection, and when they were com- 

 pletely developed, the insect was allowed to dry before 

 it was presented to the microscope. Its parts were 

 consequently contracted, and lost not only their proper 

 shape, but that plumpness, and that freshness of colour 

 which they possessed when alive. 



" In the preparation, indeed, of almost every object of 

 natural history that is composed of minute and delicate 

 parts, it must be preserved by immersion in a fluid ; 

 the dissection must often be performed in the same 

 medium ; it must be freed from ail adhesive and ex- 

 traneous substances, by maceration and ablution in wa- 

 ter ; and when it has undergone these operations, it is 

 in a state of perfection for the microscope. Every sub- 

 sequent change which it undergoes is highly injurious : 

 it shrivels and collapses by being dried ; its natural 

 polish and brilliancy are impaired ; the minute parts, 

 such as the hairs and down, adhere to one another, and 

 the general form of the object, as well as the disposi- 

 tion of its individual parts, can no longer be distinctly 

 seen. 



" It is therefore a matter of considerable importance 

 to be able to examine the object when wet, and before 

 it has suffered any of these changes; and by fitting up 

 the microscope in the following manner, this may be 

 effected without even exposing the object to the 

 air. 



" The object glass of the compound microscope should 

 have the radius of the immersed surface about nine 

 times the focal distance of the lens, and the side next 

 the eye, about three-fifths of the same distance. This 

 lens should be fixed into its tube with a cement which 

 will resist the action of water or spirits of wine ; and 

 the tube, or the part of it which holds the lens, should 

 have an universal motion, so that the axis of the lens 



Micros- 

 copes. 



may coincide to the utmost exactness with the axis of Compound 

 the tubes which contain the other glasses. 



" Several small glass vessels must then be provided, 

 having different depths, from one inch to three inches, 

 and having their bottom composed of a piece of flat 

 glass, for the purpose of admitting freely the reflected 

 light which is intended to illuminate the object. The 

 fluid in which the object has been preserved, or prepa- 

 red, is next put into the vessel ; and the object itself, 

 placed upon a glass stage, or if necessary fixed to it, is 

 immersed in the fluid. The glass vessel is now laid 

 upon the arm of the microscope, which usually holds 

 the object, and the lens is brought into contact with 

 the fluid in the vessel. The rays which diverge from 

 the object emerge directly from the fluid into the ob- 

 ject glass, and therefore suffer a less refraction than if 

 it had been made from air; but the focal length of the 

 lens is very little increased, on account of the great ra- 

 dius of its anterior surface. The object may now be 

 observed with perfect distinctness, unaffected by any 

 agitation of the fluid; its parts will be seen in their 

 finest state of preservation ; delicate muscular fibres, 

 and the hairs and down upon insects, will be kept se- 

 parate by the buoyancy of the fluid ; and if the object 

 when alive, or in its most perfect state, had a smooth 

 surface, its natural polish will not only be preserved 

 but heightened by contact with the fluid. Aquatic 

 plants and animals will thus be seen with unusual dis- 

 tinctness, and shells and unpolished minerals will have 

 a brilliancy communicated to their surfaces which they 

 could never have received from the hands of the lapi- 

 dary. If the specific gravity of the substance under 

 examination should happen to be less than that of the 

 fluid, and if it cannot easily be fixed to the glass stage, 

 it may be kept from rising to the top by a piece of thin 

 parallel glass, or by a small grating of silver wire 

 stretched across the vessel. 



" The method of fitting up and using the compound 

 microscope, which has now been described, enables us, 

 in a very simple manner, to render the object-glass 

 perfectly achromatic, without the assistance of any ad- 

 ditional lens. The rays which proceed from the ob- 

 ject immersed in the fluid, will form an image of 

 it nearly at the same distance behind the lens, as if the 

 object had been placed in air, and the rays transmitted 

 through a plain concave lens of the fluid combined 

 with the object-glass. If we, therefore, employ a fluid 

 whose dispersive power exceeds that of the object- glass, 

 and accommodate the radius of the anterior surface of' 

 that lens to the difference of their dispersive powers, 

 the image will be formed perfectly free from any of the 

 primary colours of the spectrum. The fluids most pro- 

 per for this purpose are, 



Oil of cassia. 

 Oil of anise seeds. 

 Oil of cummin. 

 Oil of cloves. 



Oil of sassafras. 



Oil of sweet fennel seeds. 



Oil of spearmint. 



Oil of pimento. 



" These oils are arranged in the order of their disper- 

 sive powers ; and when those at the top of the list are 

 used, the anterior surface of the object-glass will re- 

 quire a greater radius of curvature than when those at 

 the bottom of the list are employed. Thus, in order to 

 render the object-glass achromatic, when it is made of 

 crown glass, and when the fluid is oil of cassia, the ra- 

 dius of the anterior or immersed surface, should be to 

 that of the surface next the eye as 2.5 to 1. Lest these 

 proportions should not exactly correct the chromatic 



