168 Transactions of the Roijal Microscopical Society. 



long in proportion to their thickness, for if this be so, the central 

 rays, being reflected immediately on entering near the lower edges 

 of the prisms, will strike the opposite sides and never emerge. 



It is needless to say that if the central rays are thus lost, good 

 definition is impossible. 



I have given the length and thickness of my prisms as "68 

 and '2, but it is certainly undesirable that the length should 

 exceed three and a half times the thickness. 



The very unsatisfactory results produced by the Nicol's prism, 

 when used as an analyzer in a binocular, was the primary cause of 

 the alteration in the angle at which the bodies are inclined to the 

 perpendicular. 



It was evident that the analyzing prism might be entirely 

 dispensed with by substituting for the upper reflecting prisms a 

 highly -polished and properly-worked plate of black glass, so placed 

 that the light should be reflected from its surface at the polarizing 

 angle of 56|^, and that this change could be accomplished without 

 in any way affecting the character of the instrument as an erecting 

 binocular by altering the angle at which the bodies were inclined 

 from 75° to 66^°. The box containing the upper prisms of the 

 ordinary arrangement is withdrawn when polarized light is used, 

 and the analyzing plate" substituted ; the result is improved defi- 

 nition and more light than with the ordinary Nicol. 



I have spoken of tlie box containing the upper prisms, but in 

 the original description of the instrument there was one prism 

 only ; that prism I have now divided, and the two thus formed are 

 cemented together at such an angle that the light enters and 

 emerges at right angles to the surfaces, care of course being taken 

 that the reflecting surfaces remain in the same plane. 



Still anxious further to improve the arrangement, I have 

 caused a plane mirror to be prepared by Mr. Browning, and silvered 

 by the beautiful process employed by him in the manufacture of 

 reflecting astronomical telescopes. This, like the analyzing plate 

 of black glass, can be immediately substituted for the upper jirisms 

 if desired. By its use we obviously get rid of the whole of the 

 glass, and two surfaces ; but, on the other hand, light is not totally 

 reflected from silver, as it is by interior reflexion, and metal will 

 tarnish. 



I have thought it desirable to bring these alterations before the 

 notice of the Fellows, as by the former less than one-fifth of the 

 original quantity of glass is employed in the manufacture of 

 the upper prisms, and powers up to an «th of an inch can be used 

 with facility ; whilst by the latter, in getting rid of the Nicol's 

 analyzing prism we secure perfect illumination of both fields, better 

 definition and more light. 



