Oct. r;. 1S72] 



NATURE 



495 



viz., Profs. Simon Newcomb and William Harkness. 

 These gentlemen are all thoroughly qualified, both from 

 their attainments and position, to perform the important 

 and critical duties devolving upon them ; and from their 

 varied experience and knowledge, as well as from their 

 differences of mental constitution and vision, they are 

 more likely to do their work exhaustively and with 

 thorough efficiency, than if their task had been committed 

 to the absolute care of a single individual, no matter how 

 well qualified he might have been. " In the multitude of 

 counsellors there is wisdom." 



The Commission have set about their work in a 

 thorough and business-like way, and seem determined 

 that America shall have no rival in the perfection of the 

 preparations organised for making the most of the mo- 

 mentous astronomical event. They, however, do not 

 grudge to give the world generally the benefit of whatever 

 important conclusions may result from their inquiries and 

 experiments. At a meeting of the Commission in July 

 last, it was resolved to print such papers relating to the 

 subject as might be of sufficient interest and importance. 

 The first collection of these papers lies before us, and we 

 shall endeavour to lay before our readers the gist of its 

 contents. 



The first article is a letter from Rear-Admiral Sands to 

 the Secretary of the Navy, suggesting the advisability of 

 asking Congress to appropriate the necessary funds for 

 fitting out expeditions to observe the transit. Congress, 

 it appears, in 1871 had made a small preliminary appro- 

 priation of 2,000 dols., but the Commission having de- 

 cided that the total cost of carrying out the work in a 

 fitting manner would be 150,000 dols., to be expended in 

 three annual instalments, Rear-Admiral Sands requested 

 the Secretary to procure for them the first instalment of 

 50,000 dols., which were to be almost entirely spent in the 

 construction of instruments. Judging from the indorse- 

 ment of the Secretary, it seems certain that the request of 

 the Commission has been granted. 



The next letter is from Rear-Admiral Sands to Mr. 

 Lewis, Mr. Rutherfurd requesting his advice respecting 

 the best method of applying photography to the deter- 

 mination of the relative positions of Venus and the sun 

 during the transit. Mr. Rutherfurd replies by giving a 

 detailed description of the method of solar photography 

 employed in his own observatory, describing the form 

 of photographic instrument he considers best adapted for 

 the observation of the transit. He gives directions as to 

 the construction and manipulation of the objective, the 

 tube and focus, and the camera-box, which seem to be in 

 all essential respects similar to those which have hitherto 

 been found most efficient elsewhere. His opinion as to 

 the best form of photographic instrument is, however, 

 worth quoting. Mr. Rutherfurd says (p. 13) : — 



"If the whole matter of ordering instruments for the 

 photographing of the transit of Venus were in my control, 

 with my present lights, I should have an achromatic ob- 

 jective of 5in. aperture, and 7oin. focus, in a cell which 

 would allow of the application in front of it of a lens of 

 flint glass of such curves as would shorten the focal dis- 

 tance (for photographing) to 6oin. At the proper point I 

 would place between the two distances an enlarging lens 

 so constructed that the normal image of the sun in the 

 principal focus (then about half an inch) would be en- 

 larged to two inches at the distance of ten inches from 

 the principal focus, viz., at yoin. from the objective. The 

 camera box and tube should be one tube, and the 

 focalising rack and screw should be located at the objec- 

 tive end of the tube, thus simplifj-ing the whole arrange- 

 ment, and permitting the use of braces from end to end 

 to prevent flexure ; and on taking off the photographic 

 corrector, and taking out the enlarging lens, the instru- 

 ment will be all ready for vision. On consideration I do 

 not think I would coun^,el a sm.iUer telescope than the 

 one I h:i"e r?.-ied." 



We are glad to see that Mr. Rutherfurd has consented 

 to superintend the preparatory photographic constructions 

 and experiments. 



The last and longest, and perhaps most valuable, article 

 in the pamphlet is by Prof. Newcomb, a member of the 

 Commission, " On the Application of Photography to 

 the Observations of the Transits of Venus." He speaks 

 of the two methods which may be adopted for the pur- 

 pose of observations. Of the first, which consists in 

 fixing the moment at which the planet is in contact with 

 the limb of the sun, he speaks in terms of strong depre- 

 ciation, as almost entirely untrustworthy. The second 

 method, and the one Prof. Newcomb recommends, con- 

 sists in determining the relative position of the centre of 

 the planet and the centre of the sun as often as possible 

 during the transit. He then proceeds to examine some 

 plans which have been suggested for the application of 

 photography to this purpose, and to devise the combina- 

 tion among them which he thinks most likely to lead to 

 the desired result. The objects to be attained he sums 

 up as follows : — ■ 



I. To form an image of the sun with Venus on its disc 

 of such a kind that from the outlines of the images the 

 points on the photographic plates which correspond to 

 the centre of the two discs, can be fixed with a high de- 

 gree of precision. 2. The linear distance between these 

 points being determined in millimetres, or other units of 

 length, by means of a micrometer, we must have the 

 means of deducing the angular distance to which this 

 linear distance corresponds ; or we must know the value 

 of one millimetre in seconds of arc on each part of the 

 photographic plate, and in each direction. 3. We must 

 have a fixed line of reference on the plate, from which v/e 

 can deduce the angle of position of the two centres rela- 

 tively to the circle of right ascension passing through the 

 sun's centre. 



Prof Newcomb then speaks of the necessity for the 

 greatest possible accuracy in the measures ; he thinks that, 

 considering the accuracy with which the solar parallax can 

 be found by other methods, we are justified in pronouncing 

 it necessary that the errors at no one station rise to the 

 ^J^ of the distance measured. In speaking of the size 

 of image on plate, he assumes that the photographs must 

 be taken by the "wet plate" process. As to size, he justly 

 says that the test consists in the relative sharpness of the 

 images ; if it be found that a 2-in. image can be measured 

 with twice the accuracy of a 4-in. one, it will answer an 

 equally good purpose. 



In reference to the modes of forming the solar image 

 to be photographed, he thinks the only method that can 

 be adopted is that devised by Prof. Winlock, which has 

 been in successful operation for several years at the Har- 

 vard College Observatory, and which has been indepen- 

 dently proposed by M. Faye, of the French Acaiemy of 

 Sciences. It consists in placing the telescop.; in a fixed 

 horizontal position, while the sun's rays are thrown into it 

 by a heliostat placed in front of the object-glass. After 

 enumerating several of the decided advantages which he 

 thinks it possesses, he proceeds to describe the appliances 

 and methods by which the determinations are to be made 

 in this system. What he says as to the heliostat we think 

 very valuable, and shall endeavour to give a clear abstract 

 of it. 



If the reflecting surface of the heliostat be warmed by 

 the rays of the sun, or if the two surfaces of the reflecting 

 plates are unequally heated, then (i) the position of the 

 effective optical centre of the angular value of the milli- 

 metre on the photographic plate, will be vitiated ; (2) the 

 image formed in the focus of the objective will be blurred. 

 In considering effect (i), the problem is: — two rays from 

 points in the heavens, at the angular distance y, strike the 

 reflector, whose r idias of curvature is /;, so as to m;et after 

 reflection near the optical centre of the objective ; to find 

 the difference y' between these directions after leaving the 



