ON RECENT RESEARCHES IN THE INFRA-RED SI'ECTRUM. 463 



teen centimetres aperture, and of ten metres focal length, to tlie prism or 

 grating. The prism is of rock salt, of corresponding dimensions, worked 

 (by Brashear) with the precision of, and presenting all the external appear- 

 ance of, one of flint glass. It is mounted on a massive spectro-bolometer 

 (as the instrument which supports the prism or grating used in producing 

 the spectrum is called). This instrument includes a large azimuth circle, 

 over the centre of which the prism is placed ; and it also carries the bolo- 

 meter, which registers the spectral heat. The focal length of the image- 

 forming lens, or mirror, is in tliis instrument much greater than in the 

 Hrst one used, and all parts of the apparatus are correspondingly increased 

 in size and stability. The most important and novel feature is, however, 

 the mechanical connection of the large azimuthal circle carrying the prism, 

 with a distant photographic plate, susceptible of vertical motion, and which 

 latter takes the place of the scale formerly in front of the remote galvano - 

 meter, both circle and plate being moved by the same clockwork, which is 

 of such steadiness and precision as to make the two movements, as far as 

 possible, perfectly synchronous. 



To fix our ideas, let us suppose that the slow-moving azimuthal circle 

 carrying the prism revolves through one minute of arc in one minute of 

 time, in which case the spectrum will move horizontally across the vertical 

 bolometer thread at a proportional rate. Now, if the same mechanism, 

 which causes this circular motion of the prism and of the spectrum of one 

 minute of arc in one minute of time, causes the photographic plate to move 

 vertically before the galvanometer mirror at the rate of one centimetre of 

 space in one minute of time — if there be no allowance to make for changes 

 of temperature in the prism or for like corrections ; if the mechanician 

 has done his part in such jierfection that everything works as it should — 

 it obviously follows that, under such conditions, during every second of 

 this minute a portion of the spectrum represented by the small quantity 

 of one second of arc will have glided before the bolometer thread, and that 

 during this same second the plate will have been lifted automatically 

 through one-sixtieth of a centimetre of space. 



This is one relationship of time and space in actual use here, though 

 others may be established by the use of the change-wheels with which the 

 apparatus is provided. The essential thing is that the plate shall show 

 with great precision, and even on simple inspection, not only the inflections 

 of the energy curve there written down, but the exact relative position in 

 the distant spectrum which the bolometer tliread occupied at the moment 

 it caused the disturbance. In the case assumed, for instance, if we 

 suppose that the record on the plate commences with the part of the 

 .spectrum whose angular value is 40°, then, since 1 millimetre corresponds 

 to 6 seconds of arc, and so on, the existence of an inflection on the plate at 

 30 cm. 3y'^ mm. would show that the disturbances originated at the point 

 in the spectruto corresponding to an angular measure of 40° 30' '22"-2. 



If the arm which cari'ies the bolometer is n metres long, and if the 



thread of the bolometer is metres in diameter, the angular value of the 



bolometer thread is — . At present the linear width of the bolometer 



thread is not very materially less than formerly, but it is used with a 

 longer arm, and its virtual width is accordingly less. In present actual 

 practice (to use round figures) the optical arm carrying the spectrum 



