August 16, 1895.] 



SCIENCE. 



177. 



Fig. 3. One of the first bolographs, made in six minutes. A curve where the abscissae depend upon 

 the Tvave-lengtbs o£ the undulations; and each ordinate is the amount of energy of the wave-length represented 

 by the oorresponding abscissa. C, B and A are the visible Fraunhofer lines; [jciTj <i>, -f, H, and Wj u.^ are absorp- 

 tion bands in the infra-red. A stands for wave-length. 



only the record is automatic. The spot of 

 light reflected from the mirror of the gal- 

 vanometer no longer falls upon a scale, but 

 upon a photographic plate which is raised 

 or lowered by a clockwork. The same 

 mechanism drives the tangent screw of the 

 spectrometer, thus slowly swinging the bo- 

 lometer through the spectrum. Now the op- 

 eration is as follows : "When all the adjust- 

 ments have been made, the reading of the 

 circle is noted at the starting point. At 

 the signal the slit is opened, and a few sec- 

 onds later the clockwork is set in motion, 

 swinging the arm and lifting the plate. So 

 long as the bolometer receives the same 

 quantity of energy the spot of light remains 

 stationary and traces a vertical line upon 

 the rising plate. If the bolometer encoun- 

 ters an absorption band it cools off and the 

 spot of light moves to one side, making a 

 break in the trace. If it encounters a warm 

 region the deflection will be in the opposite 

 direction, and so on. The bolometer sti-ip, 

 as it sweeps through the darkness beyond 

 the red, traverses regions varying in their 

 quantities of heat, and continually reports 

 its condition by the deflections of the spot 

 of light, which is recorded in an irregular 

 line upon the plate until at the signal every- 

 thing stops, and in ten minutes an energy 

 curve has been traced, better in nearly every 

 respect than Prof. Langley's first one, which 

 represents thousands of tedious observa- 

 tions. (See Fig. 3.) By this method, in a 

 few hours of good work, curves are obtained 

 which show hundreds of lines where doz- 

 ens were intimated before. (See Figs. 4 



and 5.) One must have seen it to ap- 

 preciate the fascination of watching that 

 simple spot of light and seeing in one's 

 thoughts that little strip climbing up the 

 heights of energy mountains only to plunge 



Fig. 4. Three bolographs of the infra-red group 

 par showing how well different records agree, even in 

 detail. 



into a cold abyss uj)On the other side, abso- 

 lutely unerring, overlooking no trifling hil- 

 lock, overestimating no lofty peak. 



When desired, such energy curves can be 

 converted into ' line-spectra,' similar to the 

 photographs of the visible Fraunhofer lines. 

 Such a line-spectrum, combined with Schu- 

 mann's photographs of the ultra-violet and 

 Rowland's of the visible spectrum, upon 

 Rowland's scale, would give us a radiant 

 energy spectrum about six hundred feet 



