CAMERA LUCIDA. xix 



Mr. Shadbolt, and is constructed of a solid cylinder of glass terminating above in a solid 

 cone, the surface of which has the form of a parabola, and replaces the silver reflecting 

 surface. 



Brooke's Rejiecting Aj^paratus. — The purpose of this is to illuminate objects by reflected 

 light, so that they can be examined ^^'ith the highest powers. It consists of two parts ; the 

 first is essentially the same as the apparatus proposed by Mr. Wenham. The second 

 consists of a small, flat, circular metallic mirror (a flat lieberkuhn), perforated to admit the 

 lower end of the object-glass, upon which it slides, and so arranged that the reflecting surface 

 is in the same plane as the lower sm-face of the object-glass. When in use the light 

 is reflected by the parabolic surface upon the plane reflector, and thence upon the upper 

 surface of the object. 



A number of points in regard to the colour of objects, distinction of pigment-granules 

 from minute air-bubbles, &c., may be easily decided by this apparatus. In questions of 

 elevations or depressions of surface, the light should only be admitted on one side of the 

 tube (for which there is a special contrivance), so that it may proceed to the object obliquely 

 from one side only ; and the conclusions must be based upon analysis of the formation and 

 arrangement of the shadows, and not upon the general appearance, because it is well known 

 that objects, or parts of them, usually appear larger and more prominent in proportion to the 

 amount of light reflected by them to the eye. Hence, for instance, little depressions, which 

 are in fact extensions of sm'face, by reflecting more light than the surrounding flat or nearly 

 flat surfaces, would appear very brilliant and luminous, and thus resemble elevations. If 

 this apparatus be used without due regard to these points, it will be worse than useless. 



It has not, however, received the attention its importance deserves. It will probably form 

 a cheap substitute for an object-glass of large aperture, and to a great extent free us from the 

 fallacies connected with the examination of objects b}^ transmitted light. 



Camera Lucida, and steel disk or Mirror of Sommering.— One of these is requisite for 

 drawing from the microscope. The camera lucida resembles that commonly used in sketch- 

 ing landscapes, &c., but is provided wdth a fitting adapting it to the eyepiece. The mirror 

 of Sommering is a plane muTor of pohshed steel, less in diameter than the pupil of the eye, 

 supported opposite the focus of the eyepiece by a small steel arm, attached to a split ring 

 which grasps the eyepiece by its spring action. There is one disadvantage in regard to the 

 evepiece of Sommering, viz. that it inverts the image of objects, which the camera does not. 

 WTien either of these is used, the body of the microscope must if possible be placed hori- 

 zontally, and the axis of vision be directed perpendicularly ; the image of the object w^ill 

 then be seen upon the table, and may be traced with a pencil. If there be no joint to the 

 microscope, so that the body is fixed perpendicularly, the paper upon which the object is to 

 be sketched must be fixed to an upright drawing-board placed in front of the microscope ; 

 but this is a very inconvenient arrangement. A more advantageous method than the latter, 

 consists in placing a box or board upon which the paper is to be laid, close to and at a level 

 with the stage, on its right side; on then looking at the object through the microscope with 

 the left eye and at the paper upon the box with the right simultaneously, the image of the 

 object may be traced as before ; here the camera is not requisite, but considerable practice 

 is required in applying this method, and it is not so satisfactoiy as that first described with 

 the camera. In using the camera, it must be remembered that the size of the object will 

 appear greater as the distance between the eyepiece and table is increased ; hence it is best 

 always to place the microscope in one and the same position when about to use it for drawing, 

 so that the extent to which the objects are magnified by the same power may always be the 

 same. The pin mentioned at page xiii is invaluable for this purpose. 



In using either the camera or the mirror of Sommering, the eye must be kept exactly in 



