Camera Lucida, as shown in Fig. 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 LIICIDA. 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 diatomacem as tests for the separating power of his objectives : for a description of these tests we must refer the reader to Smith's Diatomacem.' 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 I'odnra 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 Pig. 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 Liccernal 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 eiad, 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 frAn 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 lamps may be 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 place 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 instrument, one person may, with a pencil, draw on the glass, or on tracing-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.