PHYSICS. 237 



er. Behind the collimating lens is a Wollaston prism with 

 its refracting edge perpendicular to the slits. By this means 

 the two beams emanating from the same slit are deviated in 

 contrary directions with reference to the length of this slit ; 

 and, for any convenient distance of the collimator from the 

 prism, the two mean rays, each proceeding from one of the 

 slits, and polarized in rectangular azimuths, are exactly jux- 

 taposed throughout their length. Traversing the spectrum 

 apparatus, they give two spectra, in which the lines of the 

 one will be the prolongation of those in the other. A Nicol 

 prism is used in order to equalize the intensities to be com- 

 pared, placed between the Wollaston prism and the spectrum 

 apparatus. By a previous calibration with the solar spectrum, 

 using the same light for both slits, when the extinction coeffi- 

 cients are of course equal, the unit of intensity is determined. 

 The substance to be examined is placed between the Wollas- 

 ton and the spectrum prism, and the equality azimuth for 

 each color measured; the ratio of the extinction coefficients 

 is then easily calculated. 



Herschel has proposed a simple form of scale for pocket 

 spectroscopes. The slit plate is removed, and in its place is 

 placed a disk of copper-foil having a fine slit cut through it 

 on one side of the centre, crossing which obliquely is a row 

 of twenty holes, one eightieth of an inch apart, five being 

 on one side of the slit and fifteen on the other, perforated in 

 the copper, the upper and lower holes being level with the 

 top and bottom of the slit. Viewed by sodium light the slit 

 appears bright, and the punctures appear as a series of yel- 

 low dots. They are placed obliquely, so that their spectra 

 in white light may not overlap and confuse their images. 

 The curve corresponding to the spectroscope is then obtained 

 in the usual way, and the value of the points obtained in 

 wave-lengths. 



Giinther has described a simple method of reversing the 

 metallic lines by means of an ordinary gas flame. Into the 

 flame of a Bunsen burner, on the opposite side from the slit, 

 a fine platinum w 7 ire is placed, bent at a right angle, the end 

 being directed vertically upward. On the other side of the 

 flame a second wire is placed, carrying a sodium salt, for ex- 

 ample. Looking at the flame through a prism of low disper- 

 sive power, the eye sees first the sodium line as a bright yel- 



