for use in Spectroscopic Analysis, 83 



line b (^ = 5183) is used, and so on. For use with terrestrial 

 spectra of bright lines, the lithium-, sodium-, and thallium-lines, 

 the hydrogen-lines obtained from a vacuum-tube, or for faint 

 spectra the carbon-lines obtained from a Bunsen burner, furnish 

 convenient reference-lines in-sufficient number. The curves are 

 drawn on such a scale that a difference in wave-length of 100 

 tenth-metres* is represented by four inches, and one turn of the 

 micrometer-screw is represented by one inch. 



It requires 21*91 turns of the screw to bring the sodium-line 

 (5892) into coincidence with the thallium-line (5349). The 

 interpolation-curves are very regular and of slight curvature. 

 In fact the readings of this micrometer may be treated in exactly 

 the same way as the readings obtained by means of a graduated 

 arc or bifilar eyepiece, or any of the other forms of measuring- 

 apparatus. 



Within the limits allowed by the construction of the instru- 

 ment, the displacements measured by the micrometer-screw ne- 

 cessary to bring a given reference-line into coincidence with other 

 lines to be measured are proportional to the angles between these 

 lines and the reference- line as they would be obtained by a 

 telescope moving over a graduated arc. The extreme displace- 

 ment of which the upper half of the lens is capable is 0*69 of an 

 inch ; and assuming a refractive index of 1*5 for the glass of 

 which the lens is made, the radius of its surface is 23 inches. 

 The deviation of a ray of light passing through the lens in its 

 extreme position is thus that due to a prism of refracting angle 



2 sin -1 „ ; that is, about twice the angle whose sine is 0*03. 



If, now, we calculate the minimum deviation which a ray of light 

 suffers in passing through the lens at points whose distances 

 from the axis of the lens are represented by 0*01, 0*02, and 

 0'03 respectively, we obtain angles very nearly in the ratio of 

 these numbers. 



The following numbers show the deviation suffered by a ray 

 of light in passing through a glass prism (/a=1*5) of varying 



angle : — 



Angle of prism. 





Deviation. 



2 sin" 1 001 

 2 sin- 1 0-02 

 2 sin- 1 0'03 



The angles 0° 3-1' 23", 1° 

 numbers 1, 2, 3. 



6 34 22 

 1 8 46 

 1 43 12 



8' 46", 1° 43' 9" would be as the 



/1\ 10 

 * A tenth-metre is I tt\) of a 



metre, 



or the ten-millionth of a milli- 



metre. 



G2 



