169 



object* Very minute variations of surface contour may 

 by these means be rendered evident. 



Collins's Webster's Universal Achromatic Condenser 

 (fig. 100), a superior mechanical contrivance, is provided 

 with a diaphragm which so gradually shuts off all excess 

 of light, that any amount of illumination can be admitted 



Fig. 100. Collins's Webster's Universal Condenser. Shutter Diaphragm seen separately 



with the greatest nicety. This valuable addition to the 

 microscope can be fitted to any instrument with or without 

 a sub-stage arrangement. Its chief advantages are that, 

 while its performing qualities take rank with much more 

 expensive forms, it is moderately cheap, and is at once an 

 achromatic condenser, parabolic illuminator, and graduating 

 diaphragm, with polariser added. By means of a lever, the 

 central aperture can be gradually closed, without losing 

 sight of the object, and at the same time the alteration is 

 effected with a great amount of delicacy. Provided an 

 object-glass has sufficient "resolving power," its capa- 

 bilities are greatly increased by this condenser, in bringing 

 out the markings on the difficult test-objects; when in 

 connexion with the Polariscope, a great increase of light 

 is obtained so much so that, by using a polarising prism 

 with the "spot lens stop" the object appears brilliantly 

 illumined on a dark ground, with fine coloured effects. 

 When high powers are used in connexion with the polari- 

 scope, the advantage derived by such an addition to the 

 ordinary mode of illumination will be at once apparent to 

 every microscopist. 



