REPORT OF THE SECRETARY. 77 



Three of these holographs, taken from a large numher covering the same region, 

 are shown in the illustration (Fig. 1), in order that the manner of recording the 

 variations in the distribution of the energy in this j)ortion of the spectrum may be 

 shown, as well as the agreement of distinct observations made of the same region 

 at different times. 



In order to accomplish this agreement and accuracy in the curves, many changes 

 suggested by experience have had to be made in the apparatus and equipment. 

 The temporary and overcrowded quarters in which this apparatus is stored have 

 been more and more occupied as the mechanical difficulties in the work increased, 

 until now the space is totally inadequate to the needs of sucli an investigation, 

 while an even more serious difficulty is that the locality is in the neighborhood of 

 street traffic, and disturbed by continual tremors. 



B. The special holographic work which is carried on during days unfavorable for 

 taking observations iucludes the classification, detailed examination, and finally, 

 the reduction of the holographs taken into linear translations of the curves, in 

 which the final result is similar, so far as the automatic reduction processes will 

 admit, to the photograjihs of the visible part of the spectrum. 



Much attention has been given to the photographic work, which has presented 

 problems somewhat difficult of solution. Much of the photographic apparatus has 

 had to be specially designed and constructed to fit the peculiar conditions governing 

 its use, but the present process, it is believed, will meet any exigencies of the work. 



The work in this direction has kept pace with the original holographs and I hope 

 soon to be able to issue the preliminary charts covering at least a portion of the 

 region under investigation. 



C. What may be called the principal work of the Observatory of this kind during 

 the year has been the improvement of the apparatus and the instrumental conditions, 

 the lines of development being — 



(1) Toward an increase of delicacy. 



(2) Toward increased stability and accuracy. 



The galvanometer, whose relative importance in the train of apparatus may almost 

 be compared to that of the brain in the nervous system, has been perfected still 

 further than was noted in my last report. 



The utility of these improvements, however, has been, as I have already said, 

 greatly impaired by the situation of the Observatory, close to traveled streets, 

 where it is subject to disturbances of the most annoying kind. This will be better 

 understood when it is considered that the galvanometer never moves very sensibly 

 under a change of temperature on the holograph of less than a millionth of a degree ; 

 that it is on the registry of these minute changes that the whole system of discov- 

 ery rests, and that false changes can be introduced by the magnetism or the tremor 

 from passing traffic, which so embarrass the observer and enhance his labor 

 that it may be said to be almost hopeless to introduce any great further improve- 

 ment in this direction till the Observatory occupies a site free from such disturb- 

 ances, in place of its present temporary and most unsuitable quarters. These inter- 

 ruptions and disturbances, whether magnetic or seismic, have prevented, except at 

 rare and short intervals, the full use of the apparatus in its more delicate capacities. 



Apjiaratus. — The clockwork, upon which depends much of the accuracy and con- 

 sequently the final efficiency of the observations, has been continually studied and 

 improved. 



The siderostat has been under continued improvements suggested by experience, 

 but these will be better described when complete. 



The Observatory, during the year, has received from the commissioners of the Rus- 

 sian Government to the World's Fair a very large block of optically fine rock salt 

 from the mines, from which a collimatiug lens over 7 inches in diameter aud a prism 

 7 inches in height has been wrought (by Brashear), which, it is believed, are the 

 largest of their material in the world. To this prism and lens is due much of the 

 increased efficiency of the whole train of apparatus. 



