90 THE MICROSCOPE AND ITS REVELATIONS. 



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

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

 curately and scientifically compared; many colors whose impressions on 

 the eve are so similar that they cannot be distinguished, being readily 

 discriminated by their spectra. The purpose of the Micro-Spectroscope 



is to apply the spectroscopic test to 

 very minute quantities of colored sub- 



^V Jp^P SB stances; and it fundamentally con- 



"$ s ff ^P"^ sists of an ordinary Eye-piece (which 



can be fitted into any Microscope) with 

 certain special modifications. As 

 originally devised by Mr. Sorby, and 

 worked-out by Mr. Browning, the 

 Micro-Spectroscope is constructed as 

 follows (Fig. 61): Above its Eye- 

 glass, which is achromatic, and made 



Micro-spectroscope. capable of focal adjustment by the 



milled-head B for rays of different re- 

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

 two of flint-glass (Fig. 62, F F) interposed between three of crown (c c c), 

 in such a manner that the emergent rays r r, which have been separated 



by the dispersive action of the flint-glass 

 * prisms, are parallel to the rays which enter 

 the combination. Below the eye-glass, in 

 the place of the ordinary stop, is a dia- 

 ___ phragm with a narrow slit, which limits the 



Arrangement of prisms in Spectroscope ad mission of light; this Can be adjusted in 



Eye-piece. yer ^ j ca ] position by the milled-head H, whilst 



the breadth of the slit is 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 compa- 

 risons of such artificial spectra, alike with the ordinary or natural Spec- 

 trum, 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 pro- 

 duction of the spectrum of any substance through which the light re- 

 flected 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 the frame D D adapted to receive it. In either case, this 

 second spectrum is seen by the eye of the observer alongside of that pro- 

 duced by the object viewed through the body of the Microscope, so that 

 the two can be exactly compared. 



88. The exact position of the Absorption-bands is as important as that 

 of the Fraunhofer-lines; and some of the most conspicuous of the latter 

 afford fixed points of reference, provided the same Spectroscope be 

 employed. The amount of dispersion determines whether the Fraunhofer- 

 lines and Absorption-bands are seen nearer or farther apart; their actual 

 positions in the field of view varying according to the dispersion, while 

 their relative positions are in constant proportion. The best contrivance 



