MICRO-SPECTKOSCOPE 323 



Equally perfect monochromatic illumination can be obtained by 

 prismatic dispersion. 



A method of approximating to monochromatic illumination has 

 been devised by Mr. Nelson which answers admirably with an 

 ordinary ^-inch wick paraffin lamp. Briefly, the rays proceeding 

 from the radiant are passed through a slit, as in fig. 268, and 

 dispersed by a prism of glass, and by means of a second slit any 

 portion we wish may be selected from the spectrum to be used for 

 the purpose required. 



First an image of the edge of the flame is focussed upon the slit 

 by means of a bull's-eye consisting of three lenses ; next the slit is 



placed in the principal focus of a lens known as a Wray 5 x 4 R R, 







working at -z (this lens is not shown in the cut). In the parallel 

 5*6* % 



beam from this lens and close to it is placed an equilateral prism of 

 dense flint set at minimum deviation. Close to the prism is placed 



another Wray 5 x 4 R R, working at J . If a cardboard screen be 



O'D 



held at the principal focus of this lens, there will be seen a spectrum 

 brilliantly illuminated. A slit nyth inch in diameter is cut in the 

 cardboard screen, through which the required colour is allowed to 

 pass to the mirror of the microscope, thence to the sub-stage con- 

 denser. For visual work blue green is the best, but for photo- 

 graphic work blue would be chosen unless orthochromatic work 

 required a colour lower down the spectrum. 



Sorby-Browning Micro-spectroscope. 1 When the solar ray is 

 decomposed into a coloured spectrum by a prism of sufficient disper- 

 sive power, to which the light is admitted by a narrow slit, a 

 multitude of dark lines make their appearance. The existence of 

 these was originally noticed by Wollaston ; but as Fraunhofer first 

 subjected them to a thorough investigation and mapped them out, 

 they are known as Fraunhofer lines. The greater the dispersion 

 given by the multiplication of prisms in the spectroscope, the more 

 of these lines are seen ; and they bear considerable magnification. 

 They result from the interruption or absorption of certain rays in 

 the solar atmosphere, according to the law, first stated by Angstrom, 

 that ' rays which a substance absorbs are precisely those which it 

 emits when made self-luminous.' Kirchhoff showed that while the 

 incandescent vapours of sodium, potassium, lithium, <fcc. give a 

 spectrum with characteristic bright lines, the same vapours intercept 

 portions of the spectrum so as to give dark lines at the points where 

 the bright ones appeared, absorbing their own special colour, but 

 allowing rays of other colours to pass through. Again, when ordinary 

 light is made to pass through coloured bodies (solid, liquid, or 

 gaseous), or is reflected from their surfaces so as to affect the eye 

 with the sensation of colour, its spectrum is commonly found to 

 exhibit absorption bamls, which differ from the Fraunhofer lines not 

 only in their greater breadth, but in being more or less nebulous or 



1 For general information on the spectroscope and its uses the student is referred to 

 Professor Roscoe's Lectures on Spectrum An a lysis, or the translation of Dr. Schellen's 

 Spectrum Analysis, and How to use the Spectroscope, by Mr. John Browning. 



Y2 



