180 HOW TO SEE WITH THE MICROSCOPE. 



" III my first construction of such object-classes I 

 therefore required to provide means which proved so 

 suitable that I have adhered to their use to the present 

 time. 



"The first appliance was a deep plano-convex lens, cen- 

 trally mounted below the object, and having its centre 

 of curvature in the object place. Afterwards I adopted 

 a plano-cylindrically convex lens, equal to a hemisphere 

 less the thicknsss of the object-slide, which was placed 

 iji immersion contact with the base of the slide, so that 

 the object itself formed the centre of curvature of this 

 illuminating lens. Around the convex surface of this 

 central lens moved a shutter to regulate and limit the 



o 



access of light, and it was provided also with a small 

 plano-concave lens which, applied by its concave to the 

 convex surface of the larger lens by immersion contact, 

 cancelled the refracting surfaces and allowed a perpen- 

 dicular beam of light to reach the suitably immersed 

 object without refraction.* 



" The device in a more complete form is represented in 

 the annexed figure, where P is the basilar plate of the 

 whole traverse system, having a circular groove and 

 track in which the carriage, C, moves. On a projecting 

 arm, A, of the carriage, C, are mounted whatever ap- 

 pliances are to be used to modify or direct the light upon 

 the traverse lens, T, in the direction of the object at 

 the centre of the system. 



" In the figure, the concave lens, N, is shown in posi- 

 tion on the arm. Thus situated, the interior convex 



* M. M. J M May, 1873, p. 213. 



