WITH THE MICROSCOPE. 2/ 



42. Camera Lucida. The camera lucida has been applied to take 

 microscopical drawings, and has been found to succeed admirably. 

 The object appears to be thrown down upon the paper, and with a 

 little practice the observer may trace the lines with great accuracy. 

 If there should be any blueness round the edge of the field, the dis- 

 tance between the prism and the eye-glass should be increased. 



43. steel Disk. If a little steel disk be placed at an angle of 

 45 degs. with the eye-glass, it will receive the magnified image of the 

 object and reflect it upwards upon the retina of the observer. The 

 disk being smaller than the aperture of the pupil, the pencil can at 

 the same time be seen very well as it traces the image apparently 

 thrown down upon the paper beneath. The steel disk is represented 

 in pi. XIII, fig. 6 1. 



44. Neutral Tint Glass Reflector. The simplest and cheapest 

 reflector for microscopical drawing, consists of a small piece of plate- 

 glass slightly coloured, but not so dark as to prevent an object being 

 seen through it perfectly. This is also arranged at an angle of 45 

 degs. with the eye-glass ; by it the draughtsman can very easily fol- 

 low the outlines with his pencil upon the paper. This instrument is 

 represented in pi. XIII, fig. 60. 



In order to use these instruments, the microscope is arranged 

 horizontally, and the paper placed on the table, as shown in pi. XII, 



% 5 6 - 



45. Arranging Light, It is important, however, in using these 

 instruments, to arrange the light carefully. The image should not be 

 illuminated too intensely, and the paper upon which the drawing is 

 made should not be too much in the shade, or the point of the pencil 

 will not be seen distinctly. Experiment can alone decide the relative 

 intensity of the light upon the object and upon the paper, but with a 

 little practice the proper amount of illumination will be discovered. 

 The object appears to be thrown upon the paper, and its outline 

 is very readily traced. If it is to be drawn smaller, it is only neces- 

 sary to place the paper upon a stand closer to the reflector. If, on 

 the other hand, a large diagram is required, the distance must be 

 increased. By placing the diagram paper upon the floor, the object 

 can be readily traced with a long pencil. In this manner many of 

 my diagrams have been made. They must of course be accurate 

 copies of the objects themselves, and are therefore far more truth- 

 ful than diagrams copied from drawings representing microscopical 

 structure, can be. In making microscopical drawings it is usual to 

 fix the paper from the centre of the eye-lens at some arbitrary 

 distance, as 10 inches. If the distance be always the same, the 

 drawings so obtained may be compared with each other, and scales 



