1883.] On the Solar Spectrum and Solar Temperature. 329 



our method of observations in order to definitively test our conclu- 

 sions as regards intensity of absorption in the part of the spectrum 

 below the red which we had explored, and also to ascertain if our 

 deductions held good in the parts we had not explored. It thus 

 became necessary to go over the work done by Langley as far as our 

 instrumental means would allow. One of the most remarkable 

 features in it seemed to be the failure of Cauchy's formula for 

 refraction, his prismatic spectrum extending below A to a distance 

 equal to AG, whilst theoretically it should end at a distance equal to 

 AE. Our first labour was to ascertain the correctness of this by 

 means of photographs taken with the diffraction grating. Since 1880, 

 when the temporary map of the normal spectrum of the infra-red 

 from X.7000 to 10,500 was published, a large number of photographs 

 for the determination of lower wave-lengths have been taken, with 

 the result that a map as far as 15,000 can now be constructed, showing 

 all the delicate Fraunhofer lines which exist in this lower region, 

 but this extent is not sufficient for a comparison with Langley's solar 

 thermogram, which extends to A28,000. Unfortunately in our climate 

 the atmospheric conditions preclude getting any results much lower 

 than 15,000, except on very rare occasions. Advantage has been taken 

 of cold dry days to take rougher photographs, which though not so 

 defined in detail as those up to \15,000, are yet sufficiently accurate 

 to compare with Langley's map. His map is accurate to figures over 

 the 100 on the tenth-metre scale, and our photographs have the same 

 accuracy. As a result of the comparison of our photographs to 

 X22,000, with the map, we may say, that the wave-lengths agree, and 

 the failure of Cauchy's formula is confirmed. Our photographs were 

 secured by separating vertically the different orders by means of a 

 white glass prism of 30°. The coincidences of the different spectra 

 were thus readily seen, and the A, value at once ascertained. The 

 glass employed throughout our researches is the same as that used by 

 Langley, to whom one of us recommended it, and of which he speaks 

 in such high praise. The use of rock-salt for anything except for 

 purely absolute quantitative work becomes unnecessary, and for com- 

 parative work the glass is as effective and more certain. No rock- 

 salt prism which we have tried will bear the use of a collimator, the 

 lens of which is filled by the radiation ; the surfaces have never been 

 nearly as perfect as those of the glass. The principal Fraunhofer lines 

 were almost indistinguishable, unless the aperture of the collimatiug 

 lens was very largely diminished. This was the case with four 

 separate prisms, which when first tried were newly polished by an 

 optician. Professor Langley has been more fortunate than ourselves 

 in this respect, and has been able to compare his glass with rock-salt. 

 We owe, however, a debt of gratitude to Dr. Guthrie and Professor 

 Tyndall for having given us rock-salt with which to experiment, and 



