KECENT PROGRESS IN OPTICS. 129 



Paschen, who have made improvements not only in the instrument itself 

 but in the delicacy of its necessary accompaniment, the galvanometer. 

 The work of Snow (Physical Review, Vol. I. pp. 28 and 05), particu- 

 larly, on the infra-red spectra of the voltaic arc and of the alkalies, and 

 that done by him in conjunction with Rubens (Astronomy and Astro- 

 physics, March, 1S93, p. 231), on refraction through rock salt, sylvite, 

 and fluorite, exhibited the capacities of the bolometer even better per- 

 haps than Langley's previous work on the sun. But more recently 

 with the collaboration of several able assistants, and more particularly 

 the great ingenuity and mechanical skill of Wadsworth, the sensitive- 

 ness of Langley's galvanometer has been so exalted, and the bolometer 

 connected in such manner with photographic apparatus as to make it 

 an automatically controlled system, by which an hour's work now brings 

 results superior in both quantity and quality to what formerly required 

 many weeks or even months (Langley, " On recent researches in the 

 infra-red spectrum;" Report of Oxford meeting of British Association, 

 1894). ISot only is an entire solar energy curve now easily obtained in 

 a single day, but even a succession of them. It becomes thus possible 

 by comparison to eliminate the effect of temporary disturbing condi- 

 tions, and to combine results in such a way as to represent the infra- 

 red cold bands almost as accurately as the absorption lines of the visi- 

 ble spectrum are indicated by use of the diffraction grating. It will 

 undoubtedly become possible to determine in large measure to what 

 extent these bands are due to atmospheric absorption, and which of 

 them are produced by absorption outside of the earth's atmosphere. 



With the diffraction grating, supplemented by the radiomicrometer, 

 Percival Lewis (Astrophysical Journal, June, 1895, p. 1, and August, 

 1895, p. 106), has recently investigated the infra-red spectra of sodium, 

 lithium, thallium, strontium, calcium, and silver, attaining results which 

 accord well with the best previously attained by those who had 

 employed the bolometer, and which demonstrate the exceeding delicacy 

 of the radiomicrometer as an instrument of research. 



THE VISIBLE SPECTRUM. 



To follow out all the applications of the spectroscope that have 

 resulted in recent additions to onr knowledge would carry us far 

 be3 T ond the scope of a single paper. It is possible only to make brief 

 mention of a few. 



For a number of years Rowland (ibid., January to August 1895) has 

 been investigating the spectra of all the chemical elements, photo- 

 graphing them in connection with the normal solar spectrum, and 

 reducing them to his table of standards, which is now accepted every- 

 where. The work is of snch magnitude that years more must elapse 

 before its completion. It now includes all wave lengths from 3,722 to 

 7,200, and of these the list already published extends as far as wave 

 lengths 5,150, or from ultra violet nearly to the middle of the green. 

 SM 95 9 



