OF ARTS AND SCIENCES. 241 



they are therefore prolonged, as horizontal dotted lines, to the 

 farthest limit at which their spectra could be clearly traced. Each 

 curve is designated by a letter, viz.: — St., standard; G, gas; C, 

 candle; L, lime; Mo, moon ; E, electric; Mg, magnesium ; Su, sun. 

 The positions of the chief solar lines are also marked for convenience 

 of reference. It will be noticed as a curious fact, that the lines a, 

 C, D, E, a point between F and G and the line II, are almost ex- 

 actly equidistant; the greatest difference being in the case of C, — 

 .9 mm. on the present scale. B is just midway between a and C, G 

 midway between the missing line and H. 



The following lights were measured in the same manner as the 

 lime light. I shall therefore give only a synopsis of my observations 

 on them. 



Gas Light. 



This is probably the easiest of all the lights to measure, on account 

 of the steadiness and uniformity of its flame. An Argand burner was 

 employed, burning about 5 ft. 8 per hour. It will be seen that it is 

 considerably bluer than the standard, containing 25 per cent more 

 violet. This probably comes from the bluer portions of the flame, 

 which are generally supposed not to give off much light. It has been 

 the custom in constructing gas-burners to suppress these portions as 

 much as possible, but it may be that what a flame thus gains in bril- 

 liancy it loses in whiteness. 



The following mean readings were obtained: — 



14.2 12.2 12.0 10.9 



74 



R 



690 

 726 



~36 



" Intrinsic" refers in all cases to the brightest part of the flame. 

 vol. xv. (k. s. vii.) 16 



