May 12, 1887] 



NATURE 



31 



the actions of one element on another rather than abandon the 

 assumption that chemical affinity acts in definite units. 



Portobello, April 28. Wm. Durham, 



[Without discussing the general question, I may point out that 

 unfortunately we are at present unable to base any argument on 

 the thermal behaviour of elements, as the fundamental values 

 are entirely unknown : we do not know, for example, what 

 amount of heat would be given out on combination of H and 

 CI ; the value deduced for IIj.Clj by Thomsen being the alge- 

 braic sum of several values, some of which are negative, some 

 positive. — H. E. Armstrong.] 



The Spherical Integrator. 



I FIND that my name has been alluded to in a letter by Prof 

 Hele Shaw, in your last number (vol. xxxv, p. 581). 



I shall be glad if you will kindly permit me to state that the 

 idea of reducing the moment of inertia of the sphere in a spherical 

 integrator, by making it hollow, occurred to me while abroad in 

 Algeria. An account of the modified form is given in the Phil. 

 Mag., August 1886, p. 147. I now find, from a letter from Prof 

 Shaw, of this month, that exactly the same method of dealing 

 with the difficulty had occurred to him. At the end of Prof 

 Shaw's letter in your last issue the following words are used : 

 " Now in the ' sine ' form, of which this integrator is an example, 

 this pin should move in the arc of a circle, and it would be 

 interesting to know if approximately correct results have been 

 obtained with what is in some respects a more convenient 

 device." From this it would appear that the principle of the 

 instrument is not correct. This morning I received a post-card 

 from Prof Shaw in which he writes that he had misunderstood 

 the diagrammatic outline in the Phil. Mag. His words are : "You 

 are quite right as you use it ; I was thinking of a contrivance in 

 which the sphere and frame move together." With respect to 

 M. Ventosa's letter on the subject, in your paper of a month ago, 

 (p. 513) in which he speaks very favourably of the method of using 

 a hollow sphere, although M. Ventosa used a spherical integrator 

 in a certain form of anemometer at an early date, yet I think 

 that all who have seen and read Prof. Shaw's work will admit 

 that he has expanded the use of the spherical integrator and its 

 mathematical importance in a way which is both masterly and 

 original. Fredk. Smith. 



28 Norham Gardens, Oxford, April 25. 



THE HENRY DRAPER MEMORIAL.^ 



DR. HENRY DRAPER, in 1872, was the first to 

 photograph the lines of a stellar spectrum. His 

 investigation, pursued for many years with great skill and 

 ingenuity, was most unfortunately interrupted in 1882 by 

 his death. The recent advances in dry-plate photography 

 have vastly increased our powers of dealing with this 

 subject. Early in 1886, accordingly, Mrs. Draper made 

 a liberal provision for carrying on this investigation at 

 the Harvard College Observatory, as a memorial to her 

 husband. The results attained are described below, and 

 show that an opportunity is open for a very important 

 and extensive investigation in this branch of astronomical 

 physics. Mrs. Draper has accordingly decided greatly to 

 extend the original plan of work, and to have it conducted 

 on a scale suited to its importance. The attempt will be 

 made to include all portions of the subject, so that the 

 final results shall form a complete discussion of the con- 

 stitution and conditions of the stars, as revealed by their 

 spectra, so far as present scientific methods permit. It 

 is hoped that a greater advance will thus be made than 

 if the subject was divided among several institutions, 

 or than if a broader range of astronomical study was 

 attempted. It is expected that a station to be established 

 in the southern hemisphere will permit the work to be 

 extended so that a similar method of study may be 

 applied to stars in all parts of the sky. The investiga- 



' " First Annual Report of the Photographic Study of Stellar Spectra." 

 Conducted at the Harvard College Observatory." Edward C. Pickering, 

 Director. With Plate. (Cambridge : John Wilson and Son, University 

 Press, 1887.) 



tions already undertaken, and described below more in 

 detail, include a catalogue of the spectra of all stars north 

 of - 24° of the sixth magnitude and brighter, a more 

 extensive catalogue of spectra of stars brighter than the 

 eighth magnitude, and a detailed study of the spectra of 

 the bright stars. This last will include a classification of 

 the spectra, a determination of the wave-lengths of the 

 lines, a comparison with terrestrial spectra, and an 

 application of the results to the measurement of the 

 approach and recession of the stars. A special photo- 

 graphic investigation will also be undertaken of the 

 spectra of the banded stars, and of the ends of the 

 spectra of the bright stars. The instruments employed 

 are an 8-inch Voigtiander photographic lens re-ground by 

 Alvan Clark and Sons, and Dr. Draper's ii-inch photo- 

 graphic lens, for which Mrs. Draper has provided a new 

 mounting and observatory. The 15-inch refractor be- 

 longing to the Harvard College Observatory has also 

 been employed in various experiments with a slit spectro- 

 scope, and is again being used as described below. Mrs. 

 Draper has decided to send to Cambridge a 28-inch re- 

 flector and its mounting, and a 15-inch mirror, which is 

 one of the most perfect reflectors constructed by Dr. 

 Draper, and with which his photograph of the moon was 

 taken. The first two instruments mentioned above have 

 been kept at work during the first part of every clear 

 night for several months. It is now intended that at 

 least three telescopes shall be used during the whole 

 night, until the work is interrupted by daylight. 



The spectra have been produced by placing in front of 

 the telescope a large prism, thus returning to the method 

 originally employed by Fraunhofer in the first study of 

 stellar spectra. Four 15° prisms have been constructed, 

 the three largest having clear apertures of nearly 11 

 inches, and the fourth being somewhat smaller. The 

 entire weight of these prisms exceeds a hundred pounds, 

 and they fill a brass cubical box a foot on each side. 

 The spectrum of a star formed by this apparatus is 

 extremely narrow when the telescope is driven by clock- 

 work in the usual way. A motion is accordingly given 

 to the telescope slightly differing from that of the earth 

 by means of a secondary clock controlling it electrically. 

 The spectrum is thus spread into a band, having a width 

 proportional to the time of exposure and to the rate of 

 the controlling clock. 



This band is generally not uniformly dense. It exhibits 

 lines perpendicular to the refracting edge of the prism, 

 such as are produced in the field of an ordinary spectro- 

 scope by particles of dust upon the slit. In the present 

 case, these lines may be due to variations in the trans- 

 parency of the air during the time of exposure, or to in- 

 strumental causes, such as irregular running of the driving 

 clock, or slight changes in the motion of the telescope, 

 resulting from the manner in which its polar axis is 

 supported. These instrumental defects may be too 

 small to be detected in ordinary micrometric or photo- 

 graphic observations, and still sufficient to affect the 

 photographs just described. 



A method of enlargement has been tried which gives 

 very satisfactory results, and removes the lines above 

 mentioned as defects in the negatives. A cylindrical 

 lens is placed close to the enlarging lens, with its axis 

 parallel to the length of the spectrum. In the apparatus 

 actually employed, the length of the spectrum, and with 

 it the dispersion, is increased five times, while the breadth 

 is made in all cases about 4 inches. The advantage 

 of this arrangement is, that it greatly reduces the difficulty 

 arising from the feeble light of the star. Until very lately, 

 the spectra in the original negatives were made very 

 narrow, since otherwise the intensity of the starlight 

 would have been insufficient to produce the proper de- 

 composition of the silver particles. The enlargement 

 being made by daylight, the vast amount of energy then 

 available is controlled by the original negative, the action 



