ACCESSORY APPARATUS. 89 



reflected upon the lower surface at the point K, and thence to the point 

 L upon the vertical face c D G H, and lastly, at the point M, upon the 

 other vertical face D E F G; from which the image normally and com- 

 pletely erected, is again sent back, to issue by the superior surface upon 

 which the eye-glass is placed. All the reflections are total except the first 

 at i; and the loss of light is far less than would be anticipated. The ob- 

 liquity which this Prism gives to the visual rays, when the Microscope is 

 placed vertically for dissecting or for the examination of objects in fluid, 

 is such as to bring them to the eye at an angle very nearly corresponding 

 with that at which the Microscope may be most conveniently used in the 

 inclined position ( 41, in.); so that, instead of being an objection, it is 

 a real advantage. 



87. Sorby- Browning Micro-Spectroscope. 1 When the Solar ray is 

 decomposed into a colored spectrum by a prism of sufficient dispersive 

 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 thor- 

 ough investigation, and mapped them out, they are known as Fraun- 



Nachet's Erecting-Prism. 



hofer- 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 pre- 

 cisely those which it emits when made self-luminous." Kirchhoif showed 

 that while the incandescent vapors of Sodium, Potassium, Lithium, etc., 

 give a spectrum with characteristic bright lines, the same vapors intercept 

 portions of white light, so as to give dark lines in place of the bright 

 ones, absorbing their own special color, but allowing rays of other colors 

 to pass through. Again, when ordinary light is made to pass through 

 colored bodies (solid, liquid, or gaseous), or is reflected from their sur- 

 faces, so as to affect the eye with the sensation of color, its spectrum is 

 commonly found to exhibit absorption lands, which differ from the Fraun- 

 hofer lines, not only in their greater breadth, but in being more or less 

 nebulous or cloudy, so that they cannot be resolved into distinct lines by 

 magnification, while too much dispersion thins them out to indistinct- 



1 For general information on the Spectroscope and its uses, the student is re- 

 ferred to Professor Roscoe's " Lectures on Spectrum Analysis," or the translation 

 of Dr. Schellen's "Spectrum Analysis." 



