822 SUMMARY OF CURRENT RESEARCHES RELATING TO 



I 



the reflectors. It is evident that by the use of reflectors in instru- 

 ments of this class, the reflecting surfaces may be larger, and the 

 distance between them greater than if a prism were used. 



The above are only examples of the application of silver films to 

 a particular class of instrument ; it is evident they ofier great facility 

 for giving this class of instrument its maximum development. It is 

 obvious also that silver films are applicable with advantage in many 

 other cases where prisms are used at present, particularly where it 

 is desired to divide a beam of light into two ; e. g. if Fig. 183 be turned 

 upside down, and the two eyes of the observer be in the place of the 

 arrow and the dotted line, the diagram represents an arrangement 

 suitable for a non-stereoscopic binocular Microscope, the inclination 

 between the mirrors being varied to suit the distance between the 

 eyes ; the loss of light in such an arrangement would be very little, 

 and the brilliancy of the two images might be rendered very nearly 

 equal." 



Apparatus for Examining Different Spectra. — Whilst the 

 diffraction spectra can readily be seen at the back of the objective by 

 the unassisted eye on removing the eye-piece, they are better defined 

 and their exact arrangement is much better observed by some addition 

 to the Microscope in the form of either an auxiliary Microscope or a 

 telescope. 



In the first case, an objective of low power, screwed into the draw- 

 tube, forms, with the eye-piece, a compound Microscope, which can be 

 focussed to the plane of the diff"raction spectra. In the second and 

 more convenient arrangement a suitable combination of lenses is 

 placed above the eye-piece, as in Ross's " centering glass," forming a 

 telescope which can be focussed upon the diffraction spectra in the 

 same manner, and easily removed to substitute other eye-pieces. This 

 plan is the one used for some time by Mr. Ingpen, whose apparatus 

 consists of a Eamsden " positive " eye-piece (formed of two plano- 

 convex lenses of equal focus with the plane surfaces turned outwards) 

 of ^ or |-inch focus, sliding in a short piece of tube over the lowest 

 power Huyghenian eye-piece. The quality of the illumination, its 

 centricity or obliquity, &c., can also be readily observed in this 

 manner. 



Sorby's Binocular Spectroscope. — This instrument (Fig. 184), 

 made by Messrs. Beck, was originally described in Proc. Roy. Soc, 

 XV. (1867) p. 433, but has not hitherto been figured. It can be 

 used with the binocular Microscope, and for many purposes is superior 

 to the ordinary micro-sijectroscope, and gives a larger disper- 

 sion. It consists of the following parts (taken from Messrs. Beck's 

 description) : — 



1st. An object-glass A, specially arranged, screwing into the tube 

 of the Microscope by the outside screw B. 



2nd. A series of compound dense glass prisms C, fitting imme- 

 diately over the object-glass A. 



3rd. A tube D, moving up and down upon that holding the prisms 

 by means of rack and pinion E, and carrying the following : — 



