MICROSCOPE. 



MICROSCOPE. 



642 



extremity, I, the objective is to be screwed on. K L M, is the mechanical 

 stage upon which the objects under microscopic examination are placed 

 the milled head marked K giving it a vertical motion, and that inarkec 

 L a lateral one. This stage has also a rotary motion, concentric wit! 

 the axis of the compound tube H G I, which enables the observer to 

 examine an object in any position of rotation, keeping the object in 

 the field of view. Fitted to the secondary .stage, N, is represented a 

 system of diaphragms employed to regulate the diameter of the pencil 

 of light reflected from either the plane or concave surface of the 

 mirror o. The diaphragms can be removed, and a variety of appara- 

 tus substituted. The regulation of the distance between the illumi- 

 nator and the object under examination is effected by means of the 

 milled head P. Q is one of two milled heads which give motion in 

 rectangular directions to the secondary stage, for the purpose of caus- 

 ing its axis to coincide with that of the compound tube. The milled 

 head marked R communicates a rotary motion to the tube of the 

 secondary stage. In using the higher powers, the instrument is only 

 approximately adjusted by the milled head F, the rack with which it 

 communicates being called the coarse adjustment : the line adjust- 

 ment is effected by means of the graduated milled head s, which is con- 

 nected with a screw and lever acting on the optical arrangement ^ of 

 an inch for each revolution of the milled head s. The milled head T is 

 used to clamp the arm to the bar of the instrument. This description 

 though brief, will be found sufficiently lucid to convey to the mind of 

 the reader correct notions of the mechanical arrangements of the com 

 pound microscope. 



Most of the directions for viewing and illuminating objects, given in 

 reference to the simple microscope, are applicable to the compound. 

 An argand lamp placed in the focus of a large detached lens so as to 

 throw parallel rays upon the mirror, is the best artificial light ; and 

 for opaque objects the light so thrown up may be reflected by metallic 

 specula (called, from their inventor, Lieberkhuus) attached to the 

 object-glasses. 



On the proposal of Sir David Brewster and M. Dujardin, the 

 Wollaston condenser was achromatised with very excellent effect by 

 constructing it with three pair of achromatic lenses, instead of the 

 single lens before described. Subsequent improvements have been 

 effected by Mr. Gillett, and a modification of the form adopted by that 

 gentleman has been constructed by Messrs. Powell and Lealand 

 Other modes of illumination have also been adopted, such as the 

 prisms of Amici, Nachet, and Powell, Nobert's circular condenser, 

 Ron's spotted lens for dark ground illumination, &c. 



The erector commonly applied to the compound microscope con- 

 sists of a pair of lenses acting like the erecting eye-piece of the 

 telescope. But this, though it is convenient for the purpose of 

 dissection, very much impairs the optical performance of the instru- 

 ment. 



For drawing the images presented by the microscope, the best 

 F j 2 3 apparatus consists of a mirror M (Pig. 23), com- 



posed of a thin piece of rather dark-coloured glass, 

 cemented on to a piece of plate-glass inclined at 

 an angle of 45 in front of the eye-glass E. The 

 light escaping from the eye-glass is assisted in its 

 reflection upwards to the eye by the dark glass, 

 which effects the further useful purpose of ren- 

 dering the paper less brilliant, and thus enabling 

 the eye better to see the reflected image. The 

 lens L, below the reflector, is to cause the light 

 from the paper and pencil to diverge from the 

 same distance as that received from the eye- 

 glass ; in other words, to cause it to reach the 

 eye in parallel lines. 



Dr. Wollaston's Camera Lucida, as shown in Ply. 24, is sometimes 

 attached to the eye-piece of the microscope for the 

 same purpose. In this instrument the rays suffer 

 two internal reflections within the glass prism, as 

 will be seen explained in the article CAMERA 

 LUCIDA. In this minute figure we have omitted 

 to trace the reflected rays, merely to avoid con- 

 fusion. 



Of late years, the microscopist has availed himself 

 of markings on the frustules of the diatomaceEO as 

 tests for the separating power of his objectives : 

 for a description of these tests we must refer the 

 reader to Smith's ' Diatomaceae.' Although many 

 objectives possessing large angles of aperture are 

 capable of discovering markings upon diatomes, yet, for definition, the 

 employment of a well-marked scale of Podura is advisable. When 

 circumstances admit of it, opticians test their objectives by means 

 of a disc of light reflected from a small globule of mercury ; this 

 test, however, can be depended upon only when employed by the 

 practised and skilful optician, great experience being required to judge 

 of an objective by this means. 



For a full description of the nature and uses of the numerous 

 adjuncts to the microscope, we must refer the reader to the valuable 

 .works of Quekett, Hogg, and Beale, on that subject. 



Although the reflecting microscope is now very little used, it may be 

 expected that we should mention it. In this instrument, at Piy. 25, 



ABT8 AM) SCI. DIV. VOL. V. 



Fig. 24. 



he object o is reflected by the inclined face of the mirror M, and the 

 ays are again reflected and converged by the ellipsoidal reflector R K, 



Fig. 25. 



which effects the same purpose as the object-glass of the compound 

 microscope. It forms an image which is not susceptible of the over- 

 correction as to colour before described, and which therefore becomes 

 coloured in passing through the eye-piece. This fact, and the loss of 

 light by reflection, will probably always render the reflecting micro- 

 scope inferior to the achromatic refracting. 



The Luccrnal Microscope is a hollow pyramidal box, of wood, at the 

 smaller extremity of which is a tube carrying the usual system of 

 lenses for magnifying objects. At the larger end, which is towards the 

 observer, there are two lenses in frames ; their axes, as well as those 

 of the small lenses at the opposite extremity, being coincident with the 

 axis of the box ; and between the exterior of the two lenses and the 

 eye of the observer there is usually placed a plate of glass, rough- 

 ground on one side, which serves as a screen to receive the rays of 

 light proceeding from the object whose representation is to be viewed : 

 the object is fixed in a .small frame, as usual, and is placed in a groove 

 made for the purpose immediately beyond the tube containing the 

 system of lenses, at the small end of the pyramidal box. 



This box, the axis of which is in a horizontal position, is mounted 

 on a brass stand, which may be placed on the floor or on a table, and 

 the parts of the apparatus are capable of being correctly adjusted by 

 means of screws. The instrument was invented by Mr. George Adams, 

 a distinguished optician in London, and it received its name from the 

 images of the objects being projected on the ground-glass screen by 

 the rays of light transmitted from a lamp through the lenses, the ob- 

 servations being made by night or in a darkened room. 



The correct definition of the image depends upon the achromatism 

 of the lenses at the object end of the instrument, a subject which has 

 been treated in the previous part of this article : and therefore it will 

 be sufficient to notice here only the means employed to enlighten the 

 field of view. 



For this purpose, in general, an Argand lamp is placed beyond the 

 object, with respect to the microscope ; or, if thought necessary, two 

 or more such lumps may bo so situated : the light, after passing through 

 a hemisphere of glass, is, when an opaque object is to be viewed, made 

 to fall in a convergent state upon a small concave mirror, which is so 

 inclined as to reflect the light back upon the object ; and from the 

 different points on the surface of the latter the pencils of rays proceed 

 through the object-lenses and the box to the glass screen. 



By the refraction of the light in passing through the lenses a highly 

 magnified image of the object is formed ; and several persons may then 

 alace themselves so as to see the image on the. screen at the same time ; 

 or, by placing the eye at a small aperture in the produced axis of the 

 nstrument, one person may, with a pencil, draw on the glass, or on 

 ;racmg-paper laid over it, the figure of the object ; it being understood 

 that, by means of the proper adjusting-screws, the rays in each pencil 

 are made to unite accurately in one point on the screen. 



When the object to be viewed is transparent, the light is made to 

 fall in a condensed state upon it, after having been transmitted through 

 a convex lens, or two such ; and, from the object, the rays proceed as 

 Before to the screen through the system of lenses which constitute 

 the compound objective of the microscope, and through those at the 

 opposite extremity of the box. 



Instead of having a plate of ground-glass to receive the image, the 

 lencils of light from the object, after passing through the lenses, may 

 je allowed to fall on a board painted white or covered with white 

 japer ; the distance of the board from the instrument (about 6 or 8 

 : eet) being such that the rays in each pencil may converge to a point 

 upon it : thus there will be obtained a greatly magnified image of the 

 object, which may be observed by many spectators at one time. Such 

 i screen should have the form of a segment of a hollow sphere, the 

 ight being received on its concave surface ; for by that construction 

 ,he image will be nearly equally distinct at the centre and about the 

 margin. 



The Solar Microscope, which was invented by Dr. Lieberkhun, in 

 .738, produces a greatly magnified image of a small object in a similar 

 manner by means of the sun's light. It consists of a conical tube fixed 

 >y its base to a frame of wood ; the latter being screwed to a closed 

 window-shutter at an aperture purposely made in the latter : the tube 

 irojects into the room, which, when the observations are to be made, 

 s rendered quite dark, and is sometimes lined with black cloth. The 

 magnifying power is produced by a system of lenses contained in the 

 ;ube, as in other microscopes. On the exterior of the window is a frame 

 carrying a rectangular piece of looking-glass ; this frame is attached 

 jy a joint at one of its sides to a ring of wood or brass which is made 



