NO. 8 SAMUEL PIERPONT LANGLEY ABBOT 3I 



"All this implies that ' drift,' if not eliminated, is to be greatly reduced. Please 

 consider this ' drift,' as well as the little movements of the needle due to changes 

 in the apparatus itself, under these three heads : 



(a) Qianges due to alterations in the galvanometer. 



(b) Changes due to alterations in the bolometer. 



(c) Changes due to alterations in the battery, and all other sources. 

 " It seems quite certain that these are due largely to temperature. 



" Our object hereafter is to map the lines." 



Under his assistants, Hallock, Wadsworth, and R. C. Child, this 

 program was so far fulfilled that in the year 1894 Langley exhibited 

 at the Oxford meeting of the British Association for the Advance- 

 ment of Science a map of the infrared solar spectrum as far as a 

 wave length of 4.2 microns. This was based on automatic energy 

 curves produced by continuous photographic records of the warming 

 and cooling of the strip of the fine linear bolometer, as expressed in 

 the swings of the sensitive galvanometer. 



The present writer and his colleague, Mr. Fowle, continued this 

 mapping of the infrared solar spectrum. Volume i of the Annals of 

 the Astrophysical Observatory contained a discussion of the apparatus, 

 a map of the infrared solar spectrum containing 579 lines and bands 

 between wave lengths 0.76 and 5.3 microns, a highly accurate measure- 

 ment of the dispersion of rock salt to 5.3 microns, and various sub- 

 sidiary reports. The finest details of the infrared spectral map de- 

 pended on a decision by the observers as to whether small nicks in the 

 energy curves denoted solar or atmospheric absorption, or merely 

 accidental error from shaking or electrical disturbance. This led 

 Langley to what seemed to me the smoothest piece of dictation I ever 

 heard. Unfortunately the stenographer was inexperienced, and it lost 

 something before printing, even though Langley spent considerable 

 time over it in manuscript and proofs. It is as follows : 



" When we approach the limits of vision or audition, or of percep- 

 tion by any other of the human senses, no matter how these may be 

 fortified by instrumental aid, we finally perceive, and always must 

 perceive, a condition still beyond, where certitude becomes incertitude, 

 although we may not be able to designate precisely where one ceases 

 and the other begins. 



" This is always the case, it would seem, on the boundaries of our 

 knowledge in every department, and it is so here. 



" It is impossible, for instance, to look at the great and notable 

 deflection of a line such as A, or to the deflections corresponding 

 to yet larger bands below it, and to see these in exactly the same 



