ASTRONOMY. 391 



mination of wave-lengths in tlie invisible prismatic spectrum, and also 

 of the influence of convection currents on thermometer bulbs. The 

 work is illustrated by twenty-one plates, mostly the observation curves 

 of the different instruments, besides a map of the district of Mount 

 Whitney, showing the proposed reservation and a view of the mountain 

 camp. 



" The i)rincipal results of Professor Langley's work may perhaps be 

 briefly summarized as follows: He has shown that the invisible spec- 

 trum beyond the red extends much further than was imagined or be- 

 lieved possible ; he has detected cold spaces in this spectrum analogous 

 to the dark absorption bands and lines of the visible spectrum, and 

 has determined their wave-lengths. He has shown that all the old 

 formula for the determination of wave-lengths in the prismatic spectrum 

 failed utterly when carried into the infra-red region. The amount of 

 absorption exercised by the atmosphere has been proved to have been 

 greatly under-estimated ; it is at least double what has been usually 

 supposed, and the value of the ' solar constant ' has been increased in 

 consequence. Contrary to the theory hitherto universally held. Pro- 

 fessor Langley has shown that the infra-red rays do not suffer the most 

 absorption, but the visible rays, and especially those in the green and 

 blue, the transmissibility steadily increasing towards the extreme infra- 

 red. From this it follows that what we call white light is not ' the 

 sum of all radiations,' but that remainder of rays which has been filtered 

 down to us, and the sun, could we see it as it is, would appeal of a de- 

 cided bluish tint. Professor Langley's observations seem also to indi- 

 cate that at a certain point far in the infra-red, transmissibility through 

 the atmosphere suddenly ceases, and it would seem to follow that the 

 earth heat which fails to be radiated away through the atmosphere 

 must be lower than we have yet examined in any spectrum. The old 

 idea of a ' temperature of space' is rejected, and reasons are given for 

 concluding that the heat we receive from celestial bodies, other than 

 the sun, is practically nil. Finally, Professor Langley concludes that 

 the temperature of the earth under direct sunshine, even though our 

 atmosphere were present as now, would probably fall to 200° C. if that 

 atmosphere did not possess the quality of selective absorj)tion. To this 

 catalogue of results may be added that the volume before us gives 

 abundant evidence of the attention and care which Professor Langley 

 has given to free his work from every source of error." {Observatory, 

 September, 1885, p. 309.) 



We can only refer by title hero to a lecture on " Sunlight and the 

 earth's atmosphere," delivered by Professor Langley at the Eoyal In- 

 stitute, April 17, 1885, a full report of which has appeared in Nature 

 (vol. 32, p. 17, 40), and to his interesting and admirably illustrated 

 astronomical articles on "The New Astronomy," in the Century for 

 September, October, and December, 1884, and March, 1885. 



