Prof. H. W. Dove on a New Photometer. 23 



some feet, such a spirit-flame may be placed between the objec- 

 tive of the microscope and the photographic image, and yet the 

 dark print upon white ground is seen on looking through the 

 flame. At a greater distance it disappears, inasmuch as the 

 print is completely covered by the flame in front. 



The brightness of the moon in different phases may be deter- 

 mined in the same manner. The microscope is directed upon 

 it, and the picture is illuminated from the front by a candle. 



The ignition of a platinum wire closing a galvanic circuit, the 

 intensity of which varies, may be determined in the same 

 manner. 



Some years ago I had occasion to see experiments upon the 

 artillery-practice ground here, in which the electric light of a 

 powerful battery was compared, by means of a large concave 

 mirror, with that produced by rockets sent up for the purpose 

 of ascertaining the progress of besiegers. The comparison was 

 effected by directly looking at the place where the men were 

 drawn up. It would have been obtained with greater certainty 

 if the illumination of a white surface had been ascertained by 

 means of the photometer. 



The spark passing from the condenser of one electrical machine 

 to another gives a very distinct picture, which may be com- 

 pensated. 



Even by the discharge of a Leyden jar the writing is visible. 



The stratified light of a very beautiful Geissler's tube was 

 investigated in the same way. To the pear-shaped middle, two 

 narrow tubes were fixed which terminated in bulbs with wires 

 fused in. The blue light which surrounded the wire with its 

 narrow envelope of light was somewhat brighter than the red 

 stratified light of the nearest tubes, and almost of the same 

 brightness as that on the second leg, but much brighter than 

 that of the other pole in the bulb nearest this pole. 



The whitish stratified light illuminated the print so as to make 

 it distinctly perceptible, both when the tube was connected with 

 the RuhmkorfFs apparatus, and when it was held on the con- 

 ductor of an ordinary electrical machine. 



One of the legs of a U-shaped Geissler's tube was surrounded 

 with sulphate of quinine fluorescing under its influence. This 

 leg shone more strongly than that which was not so surrounded, 

 a proof that the apparatus is sufficiently delicate for fluorescence. 



Phosphorescent light, on the contrary, did not come up to 

 my expectations. Only in one case out of seven did I succeed, 

 with a tube which shows very brightly after insolation, in recog- 

 nizing the writing. This, it is true, was in an inadequately 

 darkened room. 



