228 



MICROSCOPE. 



Compound 

 Micros- 

 copes. 



Fig. 2*. 



placed in its anterior focus, and illuminated by two 

 candles at A and B. As the rays A fa, and B/6 cross 

 at f, the focus of parallel rays, and as the two shadows 

 of the microscopic object will be formed at a and b, as 

 it were by rays diverging fromy, the images of these 

 two shadows formed upon the retina will coincide, and 

 make only one image, so that the object J\ will ap- 

 pear perfectly distinct. If the object, however, is 

 placed either within or without the focus J] its shadows 

 being formed as it were by rays diverging from a point 

 either within or without, the principal focus_/, will not 

 coincide on the retina, but appear to form two images, 

 either overlapping each other, or completely separated. 

 If instead of two candles A,B,'we have 4, 5,or 6, we shall 

 have 4, 5, or 6 overlapping or separated images. Now, 

 as it is impossible to place the different parts of a mi- 

 croscopic object exactly in the focus /, and as every 

 lens has different foci for the differently coloured rays, 

 and even for homogeneous light, in consequence of its 

 spherical aberration, it necessarily follows, that when 

 microscopic objects are illuminated by light proceed- 

 ing from several points, the image of it upon the retina 

 must consist of a number of images not accurately co- 

 incident ; and hence it becomes of the greatest import- 

 ance that it be illuminated only from one point, and 

 not from a large surface of light, such as the sky, 

 which is equivalent to an infinite number of radiating 

 points*. 



The following rules may therefore be laid down re- 

 specting the illumination of microscopic objects, and 

 the method of viewing them. 



1 . The eye should be protected from all extraneous 

 light, and should not receive any of the light which 

 proceeds from the illuminating centre, excepting that 

 portion of it which is transmitted through, or reflected 

 from the object. 



2. Delicate microscopical observations should not be 

 made when the fluid which lubricates the cornea of the 

 observer's eye happens to be in a viscid state, which is 

 frequently the case. See Brande's Journal, vol. ii. p. 127. 



3. The figure of the cornea will be least injured by 

 the lubricating fluid, either by collecting over any part 

 of the cornea, or moving over it, when the observer is 

 lying on his back, or standing vertically. When he is 

 looking downwards, as into the compound vertical mi- 

 croscope, the fluid has a tendency to flow towards the 

 pupil, and injure the distinctness of the vision. 



4. If the microscopic object is longitudinal, like a 

 fine hair, or consists of longitudinal stripes, the direc- 

 tion of the lines or stripes should be towards the ob- 

 server's body, in order that their form may be least in- 

 jured by the descent of the lubricating fluid over the 

 cornea. 



5. The field of view should be contracted so as to 

 exclude every part of the object excepting that which 

 is under immediate examination. 



6. The light which is employed for the purpose of 

 illuminating the object should have as small a diameter 

 as possible. In the daytime it should be a single hole 

 in the window-shutter of a darkened room, and at 

 night it should be an aperture placed before an Argand 

 lamp. 



7. In all cases, and particularly when very high Solar Mi- 

 powers are requisite, the natural diameter of the light fo copes, 

 employed should be diminished, and its intensity in- """""Y""* 

 creased by optical contrivances. 



8. When a strong light can be obtained, and indeed 

 in almost every case, homogeneous light should be thrown 

 upon the object. This may be done either by decom- 

 posing the light with a prism, or by transmitting it 

 through a coloured glass, which has the property of 

 admitting only homogeneous rays. 



CHAP. III. 



On Sular Microscopes. 



The solar microscope is an instrument for represent- Solar mi- 

 ing, upon the wall of a dark room, magnified repre- croscope 

 sentations of minute objects, illuminated by the con- invented . 

 densed light of the sun. It was invented, in the year |jj r khun " 

 1738, by Dr. Lieberkhun, who, in the winter of 1739, 1733. 

 when he was in London, showed one constructed by 

 himself, to several members of the Royal Society, and 

 several opticians, particularly Mr. Cuff and Mr. Adams. 

 Lieberkhun's solar microscope had no mirror for re- 

 flecting the sun's rays into the tube ; but Mr. Cuff soon 

 saw its imperfections, and constructed one in a very im- 

 proved form. 



Mr. Cuff's solar microscope was composed of a tube, 

 a looking-glass, a convex lens, and a Wilson's micro- 

 scope. The sun's rays were directed by the looking- 

 glass through the tube upon the object, placed a little 

 before the anterior focus of the convex glass. The 

 image of the object was thrown upon a screen of white 

 paper, and its magnitude was proportional to the dis- 

 tance of the screen from the convex lens*. M. Lie- 

 berkhun afterwards adapted the solar microscope to 

 the representation of opaque objects ; but he did not 

 leave behind him any account of the method which he 

 followed. 



M. jEpinus was the first person who described an 

 apparatus for illuminating opaque objects in the solar 

 microscope. If we suppose ef, Fig. 25, to be the ob- vig. 25. 

 ject placed before the convex lens K, he attached to the 

 tube MNOP, two parallel brass plates AB, AC, one of 

 which, AB, moved round a joint at A, and could be 

 placed at any angle with AC by means of the screw C;t 

 and spring s. On the lower end of AB and below K, he 

 fixed a mirror b d, which received the rays ac from the 

 illuminating glass NP, and threw them upon the front 

 of the object ef. By turning the screw C n, these rays 

 could be reflected at pleasure upon any part of the ob- 

 ject. See Nov. Comm. Pelrop. vol. ix. p. 326. 



The solar microscope received great improvements Martin's 

 from Mr. Benjamin Martin, who has given an account solar micro- 

 of them in;his Description and use of an Opaque Solar scope. 

 Microscope, 8vo. 1774. This instrument is represented in 



Fig. 26 33. with all the parts which are used both for pig. 86 



transparent and opaque objects. In Fig. 26. it is shewn 33. 

 as fitted up for opaque objects. In Fig. 27- is repre- 

 sented that part of it, called the single tooth and pinion 



* These observations suggest a new method of finding the principal focal length of a lens LL. Having placed two candles A, B at a 

 great distance from one another, move a small object backwards and forwards, till it appears to be single. The point f, where the two 

 shadows thus coincide, will be the focal point required. The diagram suggests also the construction of a very simple microscopic micro- 

 meter, the distance of the body to be measured from f, affording a measure of its diameter. The length of the scale will increase, as AB, 

 the distance of the lights or luminous aperture, diminishes. 



f See Phil. Tram. 1740, vol xli. p. 503. It appears that the apparatus for viewing the circulation of the blood in frogs and mice 

 was invented by a Dr. Alexander Stuart. 



