324 



ACCESSORY APPARATUS 



cloudy, so that they cannot be resolved into distinct lines by magni- 

 fication, while too much dispersion thins them out to indistinctness. 

 Now, it is by the character of these bands, and by their position in 

 the spectrum, that the colours of different substances can be most ac- 

 curately and scientifically compared, many colours whose impressions 

 on the eye are so similar that they cannot be distinguished being 

 readily discriminated by their spectra. The purpose of the micro- 

 spectroscope l is to apply the spectroscopic test to very minute 

 quantities of coloured substances ; and it fundamentally consists of 

 an ordinary eye-piece (which can be fitted into any microscope) with 

 certain special modifications. As originally devised by Dr. Sorby 

 and worked out by Mr. Browning, the micro-spectroscope is con- 

 structed as follows (fig. 269) : Above its eye-glass, which is achro- 

 matic, and made capable of focal adjustment by the milled head, B, 

 there is placed a tube, A, containing a series of five prisms, two of 

 flint glass (fig. 270, F F) interposed between three of crown 

 (C C) in such a manner that the emergent rays, r r, which have 

 been separated by dispersion, leave the prisms in much the same 



direction as the immergent ray 

 entered it. Below the eye-glass, 

 in the place of the ordinary stop, 

 is a diaphragm with a narrow slit 

 which limits the admission of light 

 (fig. 269) ; this can be adjusted in 

 vertical position by the milled head, 

 H, whilst the breadth of the slit is 



FIG. 269. Micro-spectroscope. 



FIG. 270. 



regulated by C. The foregoing, with an objective of suitable power, 

 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, I, 

 carried at the side ; whilst for the production of the spectrum of any 

 substance through which the light reflected from this mirror can be 

 transmitted, it is only necessary to place the slide carrying the 

 section or crystalline film, or the tube containing the solution, in 



1 We do not make the change, lest complications should arise; but we think it 

 would be more harmonious with analogy to call this instrument the spectro-micro- 

 scope. 



