THEORY OF ETHER 155 



able position with regard to the lens. This distance is called 

 the focal length. In order to succeed with a distant object, 

 such as a vane or flag-staff, diffused light must be shut off as 

 much as possible from the lens and screen. 



99. The Formation of a Spectrum. Place a lamp in front 

 of a vertical and narrow slit, and allow the thin ray of light 

 which passes through the slit to fall upon the face of a glass 

 prism at a suitable angle which is ascertained by trial. The 

 ray is found not only to be turned aside but to yield a hori- 

 zontal ribbon of many colours, called a spectrum, which may 

 be shown upon a screen. 



By interposing a convex lens between the prism and the 

 slit, the spectrum becomes more distinct and definite ; and, by 

 viewing the spectrum through a telescope, it is enlarged. The 

 amount of refraction suffered by the ray, as well as the extent 

 to which it is dispersed or spread out into these colours, is 

 determined by the angle and material of the prism. 



The spectroscope is an instrument in which these opera- 

 tions are carried out effectively and simultaneously. The tube 

 containing an adjustable slit, and the lenses, by which the ray 

 is prevented from diffusing, is called the collimator. The other 

 tube is the telescope through which the spectrum is enlarged 

 without being otherwise altered. The telescope is first removed, 

 and the eye-piece adjusted so as to present a clear image of 

 the cross-wires, which are inserted for exactness in comparing 

 distances. The eye-piece and cross-wires are then focussed 

 together, so that a distant object yields a clearly defined image, 

 which should not vary its position with regard to the cross-wires 

 when the eye is moved from side to side. Remove the prism 

 and replace the telescope, turning it and levelling if necessary, 

 so as to view the slit directly and in the middle of the field. 

 If necessary, focus the collimator, so that the image of the slit 

 is well defined. E/eplace the prism in such a position that a 

 good horizontal spectrum is obtained, levelling if necessary. 

 The axes of the telescope and collimator, together with the 

 graduated circle to which they are attached, should now be in 

 the same plane, if the adjustments have been carefully made. 



Instead of an ordinary white flame for illuminating the 



