2/8 ACCESSORY APPARATUS 



2. Those provided for it when used in a vertical position. 

 We shall describe what we consider the most practical forms of 

 each. 



In point of antiquity Wollastoris camera lucida claims the post 

 of honour ; but to use it the microscope must be placed in a hori- 

 zontal position. Its general form is shown in fig. 216. The rays 

 on leaving the eye-piece, above which it is fixed by a collar, enter a 

 prism, and after two internal reflections pass upwards to the eye of 

 the observer. It is easy to see a projection of the microscopic image 

 with this instrument, but it is when we desire at the same time to 

 see the paper and the fingers holding the pencil that the difficulty 

 begins. The eye has to be held in such a position that the edge of 

 the prism bisects the pupil, so that one-half of the pupil receives 

 the microscopic image and the other half the images of the paper 

 and the hand employed in drawing. If this bisection is not equal, 

 too much of one image is seen at the expense of the other. This 

 was in some sense supposed to be compensated by the use of lenses, 

 as seen in the figure ; but the difficulty of keeping the eye precisely 

 in one position has caused this instrument to fall into disuse, several 



cameras being now devised free from 

 this defect. It has nevertheless one 

 special point in its favour it does 

 not invert the image, causing the 



FIG. 216. FIG. 217. Simple camera. 



right to be turned to the left, and vice versa. This is an advantage 

 the value of which we shall subsequently see. 



A simple camera was made by Soemmering by means of a small 

 circular reflector, usually made of highly polished steel, which is 

 placed in the path of the emergent pencil at an angle of 45 to the 

 optic axis, thus reflecting rays from the image upwards. The 

 instrument, though rarely used now, is shown in fig. 217, and slides 

 on to the eye-piece. The reflector must be smaller than the pupil 

 of the eye, because it is through the peripheral portion of the pupil 

 that the rays, not stopped out by the mirror, come from the paper 

 and pencil. Hence, as in the case of Wollaston's camera, the pupil 

 of the eye must be kept perfectly centred to the small reflector. 

 As there is but one reflection, the image is inverted, but not trans- 

 posed. To see the outline of the image as it is in the microscope, 

 the drawing must be made upon tracing paper, and inverted, looking 

 at it as a transparency from the wrong side. 



There is considerable variety in the experience of different 

 microscopists as to the facilitv with which these two instruments 

 can be used. The difference in all probability depends on the 



