400 S. jP. Langley — Invisible Solar and Lunar Spectrum. 



because it is now obscured by the inevitable diffusion or reflec- 

 tion of part of the neighboring brilliant light which the prism 

 ought to keep wholly away, but (owing to inevitable instru- 

 mental defects) does not. The dull glowing iron carries no 

 white light along with it, and therefore its feeble peculiarly 

 deep red is easier seen than the far stronger corresponding red 

 in the solar spectrum. 



By the aid of this analogy in the case of light, and passing 

 now to the actual case of wholly invisible radiation, I hope it 

 may be clear how the feebler heat in the lunar extreme infra- 

 red spectrum was at first recognized more easily than the 

 stronger corresponding heat in that of the sun. 



It may be asked why (if we cannot cut off the diffused heat 

 in the solar infra-red spectrum by the use of an absorbing glass) 

 we cannot put a prism in front of the slit after the plan of 

 Helmholtz. This is practically impossible here (owing to in- 

 strumental conditions which we need not now explain), unless 

 we find some way of keeping the axis of the spectro-bolometer 

 either motionless or always parallel to itself, in spite of the 

 varying direction of the rays from such a prism, and of auto- 

 matically limiting the kind of radiation to be observed in any 

 part of the spectrum, to that legitimately belonging there. 

 The following arrangement was, after various trials, adopted 

 with success. Its immediate purpose is to overcome the diffi- 

 culty which we have just explained at such length ; that is, to 

 sift out the extraneous heat which remains after the ordinary 

 action of the prism, but it can of course be used for light also. 



Description of Sifting Train. (See fig. 1.) 



Let !N" S be a massive beam, resting on two piers, and immov- 

 ably fixed in the meridian. Let AEbea second beam, mova- 

 ble on a turn-table, placed centrally beneath BT S. Tempora- 

 rily mounted on A R, and moving about with it, is the large 

 spectro-bolometer described in a previous memoir. The center 

 of its graduated circle (C) lies under the point P. Its two long 

 arms are not free to move as usual, but are constrained by 

 mechanical attachments (not here shown) to occupy the posi- 

 tions Pp, PD. 



Two large 60° prisms of the same material (pure rock salt 

 from the same mine), their faces worked with the greatest ac- 

 curacy, are placed with these equal refracting angles in opposite 

 directions, one (P) at the obtuse, the other (p) at the acute 

 angle of the parallelogram (Pp T>c), the vertices of all whose 

 angles in the mechanical construction are pivoted and connected 

 by inflexible arms, so that (both prisms being kept automat- 

 ically in minimum deviation by the attachments M, m) the 

 angle of minimum deviation (c p P) for the first prism is 



