96 



THE MICROSCOPE AND ITS KEVELATIONS. 



four small shutters, worked by as many milled -heads projecting slightly 

 beyond the flange of the eye-piece. By combining the movements of these 

 shutters in various ways, it is easy to form a series of symmetrical aper- 

 tures, bounded by straight lines, and of any dimensions required. As 

 remarked by its inventor, this Diaphragm Eye-piece may also be used to 

 isolate one out of many objects that may be on the same slide, and thus 

 to show that object alone to persons who might not otherwise distinguish 

 it. For this last purpose the Indicator of Mr. Quekett may also be used; 

 which is a small steel hand placed just over the diaphragm, so as to point 

 to nearly the centre of the field, whilst it may be turned back when not 

 required, leaving the field of view quite free. The particular object or 

 portion of the object to which it is desired to direct attention, being 

 brought to the extremity of the hand, is thus at once ' indicated ' to any 

 other observer. 



Microscope arranged with Camera Lucida, for Drawing or Micrometry. 



94. Camera Lucida and other Draivinq Apparatus. Various contri- 

 vances may be adapted to the Eye-piece, in order to enable the observer 

 to see the image projected upon a surface whereon he may trace its out- 

 lines. The one most generally employed is the Camera Lucida prism 

 contrived by Dr. "Wollaston for the general purposes of delineation; this 

 being fitted on the front of the eye-piece, in place of the ' cap y by which 

 it is usually surmounted. The Microscope being placed in a horizontal 

 position, as shown in Eig. 68, the rays which pass through the eye-piece 

 into the prism sustain such a total reflection from its oblique surface, that 

 they come to its upper horizontal surface at right angles to their previous 

 direction; and the eye being so placed over the edge of this surface as to 

 receive these rays from the prism through part of the pupil, whilst it 

 looks with the other half beyond the prism down to a white paper surface 

 on the table, it sees the image so strongly and clearly projected upon that 

 surface, that the only difficulty in tracing it arises from a certain incapa- 

 city which seems to exist in some individuals for seeing the image and 

 the tracing-point at the same time. This difficulty (which is common 

 to all instruments devised for this purpose) is lessened by the interposition 

 of a slightly convex lens in the position shown in the figure, between the 



