496 



NATURE 



\Oct. 17, 1872 



reflector. ■/ is the angle which will be measured on the 

 plate, y t he angle we want. Call, 



S, the distance between the points on which the rays 



strike the reflector. 

 A, the angle which the line joinirg the points makes 



with the plane normal to the axis of the telescope. 

 D, mean distance of the mirror from the objective. 

 iy = y' - y, the error produced by the curvature of the 



mirror in the result of the angular measurement. 

 Then, S = D sin. y sec. //. 



. 2S 2D sec. A . 



o y = — = sm. y. 



/' P 



Sec. A may be supposed to be unity. Since it is de- 

 sirable that the error of iy should not exceed 40000 of its 

 value, it is desirable that we have -''- > So,ooo ; and since 

 it is necessary that the error should certainly be within a 

 limit four times as great as this, we must have ^ > 20,000. 



It will probably be found that at most of the stations 

 the reflector can be placed within a foot of the objective. 

 If so, the limit outside of which the radius of curvature of 

 the reflecting surface will be unim.portant, will be 80,000 

 feet, and that within which it will be inadmissible will be 

 20,000 feet. 



As to the second effect, that on definition, if the curva- 

 ture of the reflector cannot be kept within the limit of 

 80,000 feet radius, or if any small deviations witliout it 

 cannot be determined with certainty, a serious and fatal 

 objection will arise to the proposed plan. The practica- 

 bility of attaining this desideratum is the first thing to be 

 determined, and it can only be determined by tiial and 

 experiment. The most necessary precaution is that the 

 reflector should be exposed to full sunlight only at the 

 moment of taking the picture. When it "is found neces- 

 sary to use the reflected li^ht for adjustment, the heat 

 rays must, as far as possible, be cut off by a blue or green 

 glass. The necessary time of full exposure of the mirror 

 need not be more than half a second, or a second at most, 

 for each picture. 



The most perfect arrangement for moving the reflector 

 would be that of the " siderostat " of Foucault, in which 

 the mirror is moved round two axes in such a manner that 

 the reflected rays remain parallel as the sun passes along 

 its parallel of declination by its diurnal motion, the change 

 due to refraction excepted. The adjustment of the reflec- 

 tor must be made so that the direction of the reflected 

 ray shall vary from that of the telescope as little as possible 

 during the transit. The motion of the mirror must be free 

 from all vibrations, and every instrument must be carefully 

 tested for this condition before being used. To avoid all 

 serious danger of vibration. Prof. Newcomb proposes that 

 no toothed wheels shall be allowed in the moving machi- 

 nei-y, but that all motion shall be communicated by fine 

 and well-oiled tangent screws. Whether the mirror 

 should be of plain glass, silvered glass, or speculum 

 metal, is a question to be settled by experiment. 



Prof. Newcomb then proceeds to give some valuable 

 suggestions as to the objective, the tube, arrangements at 

 the focal points, the exposing of the plate, determination 

 of the planet's position on the sun's disc, and the angle of 

 position. These are an admirable rc's//me of and criticism 

 on the best results that have been hitherto arrived at 

 on these points. His concluding remarks are worth 

 quoting :— " The determination of the solar parallax from 

 measures of photographs of the sun taken during the 

 transit of Venus is beset with this serious difficulty. That 

 the required clement appears only as a minute difference 

 between two comparatively long arcs, much longer, in 

 fact, than are often measured with a micrometer. In 

 order that the solar parallax may thus be determined with 



a precision exceeding that attained by other methods, it 

 is necessary that the arcs in question be measured with a 

 precision considerably exceeding any ever attained in the 

 astronomical measurement of an arc of similar length. 

 The difficulties of the operations are greatly aggravated 

 by the direction and motion of the body to be photo- 

 graphed, which require the apparatus to be mounted on 

 moving axes, and demand either an instrument of un- 

 wieldy proportions, or the use of an enlarging lens. In 

 I'rof Winlock's apparatus the diurnal motion is thrown 

 entirely upon the revolving mirror, so that all the advan- 

 tages of a fixed horizontal sun arc obtained. The appa- 

 ratus is all firmly mounted on stone piers, thus admitting 

 of exact measurement of all its parts, and avoiding all 

 danger of changing the adjustments by the photographic 

 manipulations. It seems to be that the advantages are 

 all greatly in its favour." 



We hope that the Commission will very soon be able to 

 publish an equally, if not more, interesting and valuable 

 collection of papers, containing the results of their own 

 independent inquiries and experiments. We only hope 

 that the preliminary work will be as efficiently done in 

 other countries as there is every promise of its being done 

 in the United States. 



THE "//ASSLEJ?" EXPEDITION 



■\ ^T^E are again indebted to the New York Tribune for 

 ' * the following account of the final labours and total 

 results of Prof Agassiz's expedition : — 



San Francisco, Cal., Sept. 2. — The steamer Hassh-r 

 reached Acapulco on Sunday evening, Aug. 4, and re- 

 mained 70 hours. The fishermen of the place were very 

 active, and our own scientific party were not behindhand 

 in diligence, so that these 70 hours yielded the Professor 

 as rich a harvest as he has gathered m almost any port. 

 Acapulco is a lovely Sleepy Hollow ; its quiet little bay 

 completely enclosed by beautiful mountains ; its environs 

 adorned with a profusion of tall cocoa-nut palms ; the 

 promenade from the town to the fort, half a mile distant, 

 shaded by magnificent old lime trees ; the town itself 

 clean, old-fashioned, quiet ; only three or four vessels in 

 the port. If it had not been for the heat, we should have 

 voted it the loveliest imaginable retreat. Two of the 

 vessels in port were English, and I had one or two plea- 

 sant interviews with their captains. As we were parting, 

 I mentioned to one of them that I had long wished to 

 visit England. His answer was pre-eminently English : — 

 " He thought a visit to England would be useful to me ; 

 it might remove some prejudices and hard feelings." 

 Now, as I am absolutely certain, and do positively know, 

 that I had not betrayed to him in any way or manner the 

 least shade of prejudice or hard feeling toward the mother 

 country, I must explain his remark by supposing that he 

 was himself conscious of hard feelings toward the United 

 States ; and therefore presumed that 1 felt them toward 

 England. The confidence with which an Englishman 

 applies his English foot-rule to measure the universe is a 

 very marvellous thing ; it is as if he thought that the 

 laws and customs of his little island are universal laws of 

 humanity, and he seems incapable of supposing it other- 

 wise. 



We left Acapulco August 7. The scenery as we went out 

 of the bay, passing between the islands and the main 

 land, and for several miles after emerging into the Pacific, 

 was exquisitely beautiful. The high hflls behind the 

 town reminded me of paintings which I have seen of 

 Hymettus seen from the hill of the Museum. We have 

 hardly seen on the whole voyage anything more pictu- 

 resque and beautiful. The evening closed with a magni- 

 ficent sunset. South of the sun were long streamers of 

 golden clouds, and just north of it was a •i.-'.tch of the 



