388 Transactions of the Societij. 



XXIII. — An Illuminating Traverse-Lens. By Egbert B. Tolles, 

 of Boston, Mass., U.S. 



{Read 9th April, 1879.) 



With the advent of objectives of increased interior angle of aperture, 

 the indispensableness of equivalent accessory means for the illumi- 

 nation of the object became immediately evident.* 



In my first construction of such object-glasses, 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, centrally 

 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 thickness of the object-slide, 

 which was placed in 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 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 perpendicular beam of light to reach the suitably immersed object 

 without refractiou.t 



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 

 appliances 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 position on the 

 arm. Thus situated, the interior convex and concave surfaces 

 being of no effect, the two exterior plane surfaces of the traverse 

 system constitute it a prism, and every slightest movement of this 



* See 'M.M. J.,' July, 1S71, p. 38. t Ibicl , May, 1873, p. 213. 



