SPECTROSCOPE EYE-PIECE. 93 



tory, an Eye-piece which can be applied to any Microscope. This 

 apparatus, represented in Fig. 49, fundamentally consists of an ordi- 

 nary eye-piece, provided with certain special modifications. Above 

 its Eye-glass, which is Achromatic, and capable of focal adjustment 

 for rays of different refrangibilities, there is placed a tube containing 

 a series of five prisms, two of Flint-glass (Fig. 50, F f) interposed 

 between three of Crown p IG# 5 0> 



(c c c) in such a manner that 

 the emergent rays rr, which 

 have been separated by the 

 dispersive action of the flint- 

 glass prisms, are parallel to Arrangement of prisms in Spectroscope 

 the rays which enter the com- Eye-piece, 



bination. Below the eye-glass, in the place of the ordinary stop, is 

 a Diaphragm with a narrow slit, which limits the admission of 

 light. This would be all that is needed for the examination of the 

 Spectra of objects placed on the stage of the Microscope, whether 

 opaque or transparent, solid or liquid, provided that they transmit 

 a sufficient amount of light. But as it is of great importance to 

 make exact comparisons of such Artificial spectra, alike with the 

 Ordinary or Natural spectrum, and with each other, provision is 

 made for the formation of a second spectrum, by the insertion of a 

 right-angled prism that covers one-half of this slit, and reflects 

 upwards the light transmitted through an aperture seen on the 

 right side of the eye-piece. For the production of the ordinary 

 spectrum, it is only requisite to reflect light into this aperture from 

 the small mirror carried at the side ; whilst for the production of 

 the spectrum of any substance through which the light reflected 

 from the mirror can be transmitted, it is only necessary to place 

 the slide carrying the section or crystalline film, or the tube con- 

 taining the solution, in the frame adapted to receive it. In either 

 case, this second Spectrum is seen by the eye of the observer 

 alongside of that produced by the object viewed through the body 

 of the Microscope, so that the two can be exactly compared. * 



67. Micrometric Apparatus. — Although some have applied their 

 niicrometric apparatus to the Stage of the Microscope, yet it is to 

 the Eye- piece that it may be most advantageously adapted. f The 

 Cobweb Micrometer, invented by Ramsden for Telescopes, is pro- 

 bably, when well constructed, the most perfect instrument that 

 the Microscopist can employ. It is made by stretching across the 

 field of an Eye-piece two very delicate parallel Wires or Cobwebs, 

 one of which can be separated from the other by the action of a 



* See Mr. Sorby's description of this apparatus and of the mode of 

 using it, in the " Popular Science Review " for Jan. 1866, p. 66. 



t The Stage-micrometer constructed by Fraunhofer is employed by 

 many Continental Microscopists ; but it is subject to this disadvantage, — 

 that any error in its performance is augmented by the whole magnifying 

 power employed ; whilst a like error in the Eye-piece Micrometer is 

 increased by the magnifying power of the eye-piece alone. 



