162 Thermal Radiation in Absolute Measure. [June 16, 



The embryonic development, though in its main lines very uniform 

 throughout the group of the Thoracica (Lepadidse, Verrucida?, 

 Balanidee), shows considerable variation in some respects, and the 

 variable features are the same in all the species. 



The most conspicuous variations are those which affect the pro- 

 cesses of cell division. The details of the mode of growth of the 

 blastoderm over the yolk, from the appearance of the basal plane to 

 the closure of the blastopore, and the resulting cell arrangements 

 vary indefinitely. After the closure of the blastopore, the yolk 

 endoderm cells present in their mode of division an almost equally 

 great diversity. 



The size, shape, and colour of the ova and embryos of a species 

 vary not inconsiderably. 



In size and shape the nauplii of a species vary somewhat ; but 

 no conspicuous variations occur in structure, the larvae always show- 

 ing a great amount of uniformity, even in so minute a feature as the 

 character of each bristle belonging to an appendage. Minute varia- 

 tions occur in the ornamentation of the carapace, caudal spine, and 

 tail, and (in Chthamalus) in the number of teeth at the end of the 

 labrum. 



XV. "Thermal Radiation in Absolute Measure." By J. T. 

 BOTTOMLEY, M.A., D.Sc., F.R.S. Received June 16, 1892. 



(Abstract.) 



The paper contains an account of an experimental investigation by 

 the author in continuation of researches on the same subject which 

 have been already published (' Roy. Soc. Proc.,' 1884, and Phil. 

 Trans.,' 1887). In the earlier experiments metallic wires heated by 

 an electric current were used. The loss of heat from a heated body, 

 however, depends to some extent on the form and dimensions of the 

 body, and it seemed important to experiment on the loss of heat 

 from bodies differing in form from the wires already used, and larger 

 in dimensions. 



Accordingly, two copper globes used by Mr. D. Macfarlane in 1872 

 (' Roy. Soc. Proc.,' 1872, p. 93) were employed for a new series of 

 experiments. 



After preliminary experiments (using the same enclosure which 

 Macfarlane employed, and with the surfaces of Macfarlane's globes 

 prepared in four different ways) new apparatus was constructed ; the 

 object being to experiment both with full air pressure and with 

 different amounts of exhaustion of the air, and Macfarlane's enclosure 

 being unsuitable for this purpose. 



In the arrangement adopted, the heated globes were hung at the 



