gillett's illuminator. 



57 



GILLETT S ILLUMINATOR, OR CONDENSER. 



The advantages of employing an achromatic condenser were first 

 pointed out by Dujardin, since which time an object-glass has been 

 frequently but inconveniently employed ; and more recently achromatic 

 illuminators have been constructed by most of our instrument makers. 

 Some years since, Mr. Gillett was led by observation to appreciate the 

 importance of controlling not merely the quantity of light which may 

 be effected by a diaphragm placed any where between the source of 

 light and the object, but the angle of aperture of the illuminating 

 pencil, which can be effected only by a diaphragm placed immediately 

 behind the achromatic illuminating combination. An elastic diaphragm, 

 or artificial pupil, as it might be called, was first proposed by Mr. 

 Brooke, which was shown to answer very well in a large model, and 

 produced a remarkable semblance of vital contractility ; but mechanical 

 difficulties interfered with its application, and the revolving diaphragm 

 in the instrument, now well known as Gillett's condenser, was substi- 

 tuted. It is represented in fig. 39 as manufactured by Mr. Eoss, and 

 consists of an achromatic illuminating lens c, which is 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 

 which, at an angle of about 

 25°, is a circular rotating brass 

 plate ab, provided with a coni- 

 cal diaphragm, having a series 

 of circular apertures of differ- 

 ent sizes hg, each of which in 

 succession, as the diaphragm is 

 rotated, proportionally limits 

 the light transmitted through 

 the illuminating lens. The 

 circular plate in which the 



fig. 39. Gillett's Condenser. 



conical diaphragm is fixed is provided with a spring and catch ef, 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 

 illumination, allowing only the passage of a hollow cone of light to illu- 



