186 SEC. 7. LIGHT. 



graduation and numbering, allows the ware lengths at any place to be read off 

 in micro-millimetres. 



856. Apps' Improved Gas and Electrode Holder for 

 Spectrum Analysis. Alfred Apps. 



857. Improved Automatic Chemical Spectroscope, 



invented and made by the contributor. The object glasses and 

 prisms by Chas. Owen, Optician, Strand. 



Rev. Nicholas Brady, M.A. 



Prisms with a circular face are cemented to the object glasses of the 

 collimator and telescope, the circular face of the prisms being of the same 

 size as the object glass. The base is rectangular to the surface of the object 

 glass, and the refracting angle about 30. The beam of light rendered 

 parallel by the collimating lens passes through the first half prism perpen- 

 dicularly and suffers no refraction, but is refracted on emergence from its 

 exterior face ; the refracted and dispersed beam is received on the external 

 face of a second object glass prism, suffers more refraction and dispersion, 

 and, emerging at a perpendicular incidence, is taken up and brought to a 

 focus by the object glass. In all positions the ray under examination 

 passes parallel to the base of the prisms, and therefore at the angle of 

 minimum deviation. The variation of the angle between the two prisms by 

 the motion of the tangent screw of the telescope merely brings one ray after 

 another into the field of view. ' This automatic arrangement gives a dispersion 

 equal to one dense glass prism of 50. Should greater dispersion be required 

 I have arranged a prism of 60 in the centre of the instrument, which by a 

 very simple automatic contrivance of one lever and a slot is moved by the 

 arm canying the telescope, so that any ray is still preserved at the angle of 

 minimum deviation. A further advantage of this new principle is, that with 

 these object glass prisms the field is completely filled with light, which has 

 never usually been the case, unless the prisms are extra large, and therefore 

 expensive ; and if a train of prisms be inserted their faces only require to be 

 equal for them still to entirely illumine the field : thus much light is gained, 

 and comparatively little is lost by absorption and reflection, as the surfaces 

 are fewer, so that the violet end of the spectrum is very extensive, and the 

 lines beautifully defined. 



858. Gas Lamp Apparatus for placing before the slit of 

 the spectroscope with Bunsen burners, &c., insuring the proper 

 adjustment of the platinum wires carrying the substance under 

 examination in the flame without displacing the eye from the 

 ocular of the telescope ; and also an arrangement for quickly and 

 efficiently exchanging one or both burners for either one or two 

 vacuum tubes. Rev. Nicholas Brady, M.A. 



Two photographs accompany the instrument, showing its use in two 

 different positions. 



859. Ordinary Spectroscope, arranged for the exhibition 

 of diffraction phenomena, with aperture and gratings, &c. of 

 various kinds under common and polarised light, and with the 

 means of observing the spectra of the diffracted beam. Designed 

 and made by contributor. Rev. Nicholas Brady, M.A. 



