GILLETT'S ILLUMINATOR. 163 



about equal to an object-glass of one-quarter of an inch 

 focal length, having an angular aperture of 80. This 

 lens is placed on the top of a brass tube, intersecting 



Fig. 99Gillelt's Condenser. 



which, at an angle of about 25, is a circular rotating 

 brass plate a b, provided with a conical diaphragm, having 

 a series of circular apertures of different sizes h y, each of 

 which in succession, as the diaphragm is rotated, propor- 

 tionally limits the light transmitted through the illumi- 

 nating lens. The circular plate in which the conical dia- 

 phragm is fixed is provided with a spring and catch <?/, 

 the latter indicating when an aperture is central with the 

 illuminating lens, also the number of the aperture as 

 marked on the graduated circular plate. Three of these 

 apertures have central discs, for circularly oblique illumi- 

 nation, allowing only the passage of a hollow cone of light 

 to illuminate the object. The illuminator above described 

 is placed in the secondary stage i t, which is situated below 

 the general stage of the microscope, and consists of a 

 cylindrical tube having a rotatory motion, also a rect- 

 angular adjustment, which is effected by means of two 

 screws I m, one in front, and the other on the left side of 

 its frame. This tube receives and supports all the various 



