146 



PRACTICAL TREATISE ON THE MICROSCOPE. 



Fig. 101. 



at their Intersection might pass through it; this plan was 

 very convenient and answered well with the lower powers, 

 but with the higher, the definition was not good, in conse- 

 quence of the introduction of the glass between the eye 

 and field lenses. The author was, therefore, led to the con- 

 struction of the Indicator, represented by fig. 101, which is 

 a very simple apparatus, and can 

 be applied readily to any of the 

 eye-pieces ; the lowest of these is re- 

 presented in section by fig. 101, the 

 eye-glass and field-glass being both 

 shown to be planoconvex, with their 

 plane surfaces towards the eye. Im- 

 mediately above the field-glass is seen 

 the stop or diaphragm, with an open- 

 ing in it about half-an-inch in diame- 

 ter ; between the diaphragm and the 

 upper plate of the eye-piece, a thin 

 spindle of wire is placed, having a 

 very dehcate hand, a, like that of a watch, attached to it in the 

 focus of the eye-glass. The spindle is provided at its upper 

 part with a small handle, h, for the purpose of turning it and the 

 hand just one-fourth part of a circle. When the indicator is 

 not wanted, the hand is obscured from the field of view by 

 being turned against the side of the tube, away from the aper- 

 ture in the stop ; but when required for use, it is turned over 

 the aperture, and then the field of view appears, as is shown 

 by fig. 102. Th& hand may be turned into the centre of the 

 field, and any object in particular 

 that is required to be indicated can 

 be brought by the stage movements 

 immediately opposite to the end of 

 the hand. The form of hand first 

 employed was one with a hole near 

 its free extremity ; but it was found 

 that the hght was decomposed around 

 the inner margins of the ring, this 

 led to the adoption of the form repre- 



Fig. 102. 



