1887.] 



On Photometry of the Glow Lamp. 



247 



The absence of segmentation may probably be accounted for by the 

 migratory habits of the embryo after development has commenced. 

 The fact that no sperm-morula is formed supports this view. The 

 evidence before us does not support the view that the ovum of 

 Millepora formerly contained much yolk and has subsequently lost it. 



I am inclined to believe that the Hydrocorallinae belong to a 

 separate stock of the Hydrozoa, which probably never possessed 

 medusiform gonophores. Millepora is not related to Hydractinia. 



IV. " On Photometry of the Glow Lamp." By Captain Abney, 

 R.E., F.R.S., and Major-General FESTING, R.E., F.R.S. 

 Received November 21, 1887. 



In. a paper which we read before the -Royal Society ('Roy. Soc. 

 Proc.,' No. 232, 1884) it was shown when a carbon filament or a 

 platinum wire in vacuo was gradually raised in temperature, that the 

 different rays in the visible and invisible regions of the spectrum 

 followed a law governing their intensuvv. 



In the dark region of the spectrum (below the red) if the abscissae 

 to a curve represented watts (current x potential), and the ordinates 

 the intensity of the ray under consideration, the curve sO formed 

 was hyperbolic, approaching more nearly to the parabolic form as 

 the red was approached. In the visible spectrum the parabolic curve 

 was actually reached, the vertices of the parabolas moving along the 

 axis of abscissae; the shift being greater the more refrangible the 

 rays under consideration. This implied that until a certain number 

 of watts had been expended the ray was absent. Farther, we had 

 shown in the ' Philosophical Magazine ' for September, 1883, that 

 when measured by a thermopile, 



total radiation oc (watts — constant). 



In the visible radiation of an incandescent filament in a glow 

 lamp we are only dealing, however, with a small portion of the radia- 

 tion, and therefore could not expect it to follow such a simple law as 

 that which governs total radiation. It appeared probable, however, 

 that as the intensity of any individual ray in this part of the 

 spectrum increased parabolically, the sum of all the visible rays 

 ought also to follow very closely the same form of curve, the vertex of 

 such parabola lying at some point in the axis of abscissae between the 

 vertices of the parabolas of the extreme visible rays. It likewise 

 appeared probable that when the rays of extreme refrangibility were 

 absent or in defect, as is the case when the filament is red hot, the 

 parabola would fail to represent the intensity of visible radiation. 



In the communication we have already referred to one example of 



