510 Mr. J. N. Lockyer on Spectrum- Analysis. [Nov. 27, 



with a mixture of lead, tin, and zinc constituting the rest of the alloy, the 

 behaviour of the cadmium lines was sensibly the same as in a mixture of 

 0*1 per cent of cadmium and 99*9 of tin. 



In the Mint specimens the same phenomena were observed en petit as 

 the coarser alloys showed en grand. In a gold-copper alloy y^W increase 

 in the gold made the lines shorter, and a similar increase in the copper 

 made them longer. 



In the silver-copper alloy an increase of y-gW in the silver lengthened 

 the lines, a similar increase in the copper shortened them. 



These phenomena can be explained by assuming such alloys to be 

 different physical things, and that the spark acts upon the alloy as a 

 whole as well as upon each vapour separately. 



Thus, in these Mint alloys, copper is common to both, and their melt- 

 ing-points are : — 



Gold.... 1200° (Pouillet). 



Copper . . 1200° to 1000°, the precise point not determined. 

 Silver . . 1000° (Pouillet). 



The intermediate position of copper explains the different action on 

 its lines of gold and silver. 



II. The Method of photographing Spectra adopted. 



A camera carrying a 5 x 5-inch plate and a 3-inch lens of 23 inches focus, 

 replaced the observing-telescope of the spectroscope. The lens focused 

 from 3900 to 4500 very fairly upon the plate. The beam passing through 

 collimator and prisms was, as in Mr. Rutherford's researches, very small. 

 As the electric arc in its usual vertical position gave all the lines from 

 pole to pole, the lamp was placed on its side, and the arc used in a hori- 

 zontal position, the slit being vertical. The dense core of the arc then 

 gave all the short lines in the centre of the field, the longer ones ex- 

 tending beyond them on either side. In order to obtain a scale, it was 

 resolved to photograph the solar spectrum immediately adjacent to the 

 metallic spectrum under examination. 



To effect this a portion of the slit was covered up while the solar 

 spectrum passed through the free part, and then the part used for the 

 solar spectrum was covered, while the formerly covered part was opened 

 for the metallic spectrum. This was effected by a shutter, with an 

 opening sliding in front of the slit ; a diagram of its action and form is 

 given. 



The arrangement of the spectroscope, heliostat, &c. for obtaining the 

 sun's light is described. The image of the sun was brought to a focus 

 between the poles of the lamp by an extra lens interposed between the 

 lamp and the heliostat. 



The use of the shutter enables us to compare either two or more 

 spectra upon a single plate ; or the solar spectrum may be compared with 



