GILLETT S ACHROMATIC CONDENSER. 



177 



has been frequently but inconveniently employed ; and 

 more recently much, attention has been bestowed upon 

 achromatic illuminators by most of our instrument 

 makers. It is now some years since Mr. Gillett was 

 led by observation to appreciate the importance of 

 controlling and condensing the quantity of light by 

 a diaphragm placed anywhere between the source of 

 light and the object. This he found more fully effected 

 by a diaphragm placed immediately behind the achro- 

 matic illuminating combination. Such a diaphragm is 

 represented in fig. 109, Ross's original Gillett. It con- 

 sists of an achromatic illuminating lens c, which is about 



d. 



FIG. 109. The original form ofGrilletfs Achromatic Condenser. 



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

 length, with an angular aperture of 80. This lens 

 is screwed on to the top of a brass tube, and intersect- 

 ing which, at an angle of about 25, is a circular rotat- 

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

 having a series of circular apertures of different sizes 

 h g, 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 conical diaphragm is fixed is provided with a 

 spring and catch ef. the latter indicating when an aper- 

 ture is central with the illuminating lens, also the num- 



