WITH THE MICROSCOPE. 2 19 



and thus many colours which appear almost absolutely alike can be 

 easily distinguished by their spectra. An ordinary spectroscope with 

 small dispersion would suffice to study many of the facts, and even a 

 prism and a narrow slit in a card could be employed ; but in order 

 to carry on the enquiries with entire success, it is desirable to have 

 an instrument by means of which spectra of minute quantities of 

 material can be examined, compared side by side with other spectra, 

 and measured with considerable accuracy. All these advantages are 

 secured by means of the spectrum apparatus applied to the micro- 

 scope, made according to my plan by Mr. John Browning. I have 

 indeed constructed a binocular spectrum microscope which is far 

 more convenient in chemical testing, but is not suited for the 

 examination of any substance less than T ^ of an inch in diameter. 

 I shall therefore confine myself to a description of the single eye- 

 piece arrangement as being the most simple and generally applicable. 

 Fig. 319, pi. L, shows the more important parts of the apparatus. 

 It is an eye-piece, fitting into the tube of the microscope, having the 

 upper lens (c] made achromatic. At the focal point of this lens (</) 

 is fixed the narrow slit of which fig. 320 gives, as it were, the 

 ground plan ; and this can be made broader or narrower by turning 

 the head of the screw (#*). A small rectangular prism (e) is fixed so 

 as to extend over about one-half of the slit, and reflect the light 

 coming through an aperture at (f) in the stage attached to the side 

 of the eye-piece, as shown in fig. 319. The other half of the slit 

 transmits the light passing up the main body of the microscope 

 through the ordinary object-glass. When all is properly arranged 

 and illuminated, in looking through the lens (c), a narrow line 

 of light can be seen, one-half the length of which has passed 

 through an object placed on the stage of the microscope, and 

 the other 'half through any other placed on the side stage at- 

 tached to the eye-piece ; and, if the prism (<?) has been pro- 

 perly adjusted, these two portions should appear perfectly con- 

 tinuous, without any break at their junction ; but if not properly 

 adjusted the line appears broken, and would then give false results 

 if the spectra were compared together. Care should therefore be 

 taken to see that the adjustment is correct. The analysing prism 

 (ab) is compound, and fits over the eye-piece like a long cap. It 

 consists of two rectangular prisms of flint glass, corrected for refrac- 

 tion by one rectangular prism of crown glass, and two others, with 

 angles of about 75. This combination gives direct vision, and an 

 amount of dispersion which is admirably fitted for the purpose to 

 which this instrument is applied ; since it is sufficient to divide all 

 the absorption bands seen in coloured solids and liquids, and is not 



