1 8 PRACTICAL HISTOLOGY. 



of the paper ; this is effected in this instrument by smoke-tinted 

 glasses, which fit into the prism mounting. In other cases this is 

 done by means of a blackened cardboard shade. 



(iii.) Chevalier's Camera Lucida (fig. 19). The microscope 

 remains erect, the eye-piece is removed, and in its place is inserted 



the tube of the camera. Place 

 a sheet of white paper at the 

 side of the microscope, and 

 directly under the horizontal 

 part of the camera. The in- 

 strument itself consists of 

 two tubes at right angles to 

 each other, and at the angle 

 is a prism, which reflects at 

 a right angle in a horizontal 

 direction the light coming 

 vcr ti ca lly from an object on 

 the stage. These horizontal 



rays have to pass through what is practically an eye-piece of a 

 microscope, and reach a small prism at the free end of the hori- 

 zontal part of the instrument. This prism is so arranged that the 

 rays coming from the object are reflected into the eye of an observer, 

 who, on looking through the prism, sees an image of the object upon 

 the sheet of paper on the table. The observer looks through this 

 prism, but he must so adjust his position as to bring one-half of the 

 pupil over the prism, and thus with one-half of the pupil view the 

 object on the paper, while the other half of the pupil receives the 

 rays of light coming from the pencil and the paper. The distance 

 between the eye of the observer and the paper is about 10 inches. 

 It is by no means so easy to sketch the outline of an object with 

 this camera. The eye should be protected from other rays of light 

 by means of a blackened shade. The difficulty encountered by 

 the student is to see the object and the point of the pencil at the 

 same time. 



(iv.} Camera of Malassez (fig. 20). This consists of two prisms; 

 one, the ocular prism, is fixed, while the second and larger can be 

 moved round a horizontal axis. This enables the instrument to be 

 used in two different positions of the microscope. If one is 

 examining a fluid, the microscope is erect and not inclined. The 

 instrument is fixed to the upper part of the tube of the microscope. 

 Slip the collar over the tube and insert the eye-piece. The image 

 is projected on the table to the right of the microscope ; but the 

 paper must be placed at an angle, i.e., on an inclined plane, horizontal 

 to the direction of the rays from the camera. This is necessary to 

 avoid one side of the figure being larger than the other. If it be 



