CONSTRUCTION OF THE MICROSCOPE. 



71 



perience. The Wenham prism at the common base of the bodies is fitted 

 into an oblong box, which slides through the arm that carries them; this 

 contains, in addition, a Nicol analyzing prism, and is also pierced with a 

 vacant aperture; so that, by merely sliding this box transversely until its 

 aperture comes into the axis, the instrument may be used as an ordinary 

 Monocular; or, if the analyzing prism be made to take the place of the 

 Wenham, whilst the polarizing prism beneath the stage is brought into 

 position bjr rotating the Diaphragm-plate in which it is fixed, it is at once 

 converted into a Polarizing Microscope with the disadvantage, however, 

 of not being then Binocular. It has also a 'nose-piece' carrying two 

 Objectives, by a sliding movement of which one power may be substi- 

 tuted for the other. * 



68. Swiff s Challenge Micro- 

 scope. The instrument con- 

 structed under this designation 

 by Messrs. Swift, is one of which 

 it may be fairly said that it is 

 surpassed by no other of its price 

 in the excellence of its work- 

 manship, and its suitability to 

 the general wants of the Micro- 

 scopist. The support on which 

 it is hung is extremely firm and 

 substantial without being heavy; 

 and when the limb is brought to 

 the horizontal position, resting 

 on the cross plate between the 

 two uprights, the instument is 

 still well balanced. The rack 

 and pinion movement is made 

 with oblique teeth; a construc- 

 tion which favors smoothness 

 and sensitiveness in the adjust- 

 ment, so that a l-4th inch objec- 

 tive may be f ocussed by it alone. 

 The fine adjustment is made by 

 the milled-head at the lower end 

 of the body. It is a peculiarity 

 in this instrument, which espe- 

 cially fits it for those who work 

 much with Polarized light, that 

 the analyzing prism is fitted 



into the body above the Wenham prism, in such a manner that, when 

 its fitting is drawn out (without being removed), it is completely 

 out of the way of the light-rays; whilst, when the use of the Polariscope 

 is required, the prism can be at once pushed into the body, working in 

 conjunction with the Wenham prism. This mode of mounting the 

 analyzer is found to interfere much less with the definition of the objec- 

 tive, than the insertion of it between the objective and the Wenham 



1 The price of this instrument, with Mechanical rotating Stage, two pairs of 

 Eye-pieces, two Objectives (either a 2-inch of 12, or a 1-inch of 18, with a l-4th 

 of 95), Side-Condenser on Stand, and Polarizing apparatus in Cabinet, is 19. 

 Accessories of various kinds can be readily fitted to it. A * first-class ' Binocular 

 is also constructed by the same Maker on the Jackson model. 



Swift's Challenge Microscope. 



