1868,] On Physical Observations to be undertaken in India. 



285 



too feeble to allow of it, might be valuable, and an examination of tlie 

 light for polarization with a double-image prism. 



" § 3. The climate of Mussoorie would make actinometric observations 

 at that place especially interesting. On account of the fragility of the 

 well-known instrument invented by Sir John Herschel, it is proposed to 

 employ the actinometer of Mr. Hodgkinson. At least two instruments 

 should be provided, which would allow of simultaneous observations at 

 different altitudes being taken when the services of a second observer could 

 be obtained. It seems desirable, as suggested by Mr. Hodgkinson, that 

 an actinometer should be preserved at Kew, to furnish a fixed though 

 arbitrary scale for reference, and that the instruments sent out should be 

 compared before departure with the Kew standard, and their coefficients 

 of reduction to the Kew scale determined. 



" § 4. Much attention has recently been bestowed on the atmospheric 

 lines of the solar spectrum ; and it appears to be pretty well established 

 that they are, partly at least, due to absorption by watery vapour. The 

 extreme clearness of atmosphere at Mussoorie would seem to render this 

 a desirable subject of investigation at that place, provided a clear view of 

 the sun can be obtained, either at rising or at setting, down to the horizon, 

 or at least to a very low altitude ; and such the Committee are informed 

 is the case. 



" For this purpose the observer should be provided with a spectroscope 

 of similar power to that with which Fraunhofer's or Kirchhoff's excellent 

 map of the spectrum was constructed. It is proposed to take the latter 

 as a basis on which to work, but that the observer should confine his at- 

 tention, at least in the first instance, to the region extending from the 

 extreme red to the line E. The spectrum should first be observed when 

 the sun is high, and compared with Kirchhoff's map. Should any lines 

 represented in the map not be found, and should the want not be due to 

 inferior spectroscopic power, (which may be judged of by comparing 

 neighbouring lines of similar strength with their representation in the map,) 

 such missing lines should be noted as lines probably atmospheric included 

 in the map. Then the spectrum should be observed at low altitudes of 

 the sun, and the lines seen in addition to those in the map measured and 

 drawn. Differential measures referring the additional lines to those repre- 

 sented in the map would be sufficient. Any remarkable increase of breadth 

 of a line, too great to be accounted for merely as a result of decreasing 

 illumination, and proved by direct observation, by weakening the light 

 when the sun is high, not to be referable to that cause, should also be 

 noted. The lines in the map which were missed when the sun was high 

 should now be sought for ; for if really atmospheric they would be sure 

 now to come out strongly, unless indeed they were produced by exceptional 

 atmospheric conditions. The time at which the drawings were made 

 should also be noted, from which the sun's altitude could afterwards be 

 calculated if desired; and the hygrometric condition of the ah- at the 



VOL. XVI. 2 D 



