NA TURE 



\Att^. 15, 1872 



the growth of the Scientific Study of Nature, the conception of 

 its Harmony and Unity gained ever-increasing strength. And 

 so among the most enlightened of the Greek and Roman Philo- 

 sophers, we find a distinct recognition of the idea of the Unity 

 of the Directing Mind from which tire Order of Nature proceeds ; 

 for they obviously believed that, as our modern Poet has ex- 

 pressed it — 



All are but'parts of one stupendous whole, 

 Whose body Nature is, and God the Soul. 



The Science of Modem times, however, has taken a more 

 special direction. Fixing its attention exclusively on the Order 

 of Nature, it has separated itself wholly from Theology, whose 

 function it is to seek after its Cause. In this. Science is fully 

 justified, alike by the entire independence of its objects, and by 

 the historical fact that it has been continually hampered and im- 

 peded in its search for the Truth as it is in Nature, by the 

 restraints which Theologians have attempted to impose upon its 

 inquiries. But when Science, passing beyond its own limits, 

 assumes to take the place of Theology, and sets up its own con- 

 ception of the Order of Nature as a sufficient account of its Cause, 

 it is invading a province of Thought to which it has no claim, 

 and not unreasonably provokes the hostility of those who ought 

 to be its best friends. 



For whilst the deep-seated instincts of Humanity, and the pro- 

 foundest researches of Philosophy, alike point to Mind as the one 

 and only source of Power, it is the high prerogative nf Science to 

 demonstrate the Unity of the Power which is operating through 

 the limitless extent and variety of the Universe, and to trace its 

 Continuity through the vast seiies of Ages that have been occupied 

 in its Evolution. 



SECTION A 

 mathematical and physical science 



Opening Address by the President, Mr. Warren De 



La Rue, D.C.L., F.R.S. 

 My predecessors in this chair have addressed you on many 

 sjbjects of high interest in Mathematical and Physical 

 Science : I do not contemplate passing in review the recent dis- 

 coveries in Astronomy and Physical Science, but intend to con- 

 fine myself, in the main, to Astronomical Photography ; and in 

 selecting this branch of science as the subject of this introductory 

 discourse, I think that I shall have your approval, not only be- 

 cause I have given special attention to that subject, but als» 

 because it is about to be applied to the determination of a funda- 

 mental element of our system, the solar parallax, by observations 

 of the transit of Venus in 1S74, and probably also in 18S2. 



Nothing is so lastingly injurious to the progress of science as 

 false data, for they endure often through many centuries. False 

 views, even if supported by some amount of evidence, do com- 

 paratively little harm, for every one takes a salutary interest in 

 jiroving their falseness, and when this is done the path to error 

 is closed, and the road to truth is opened at the same moment. 



It will be conceded that Photography, when applied to scien- 

 tific observation, undoubtedly preserves facts. But the question 

 has some! imes been raised. Are photographic records absolutely 

 trustworthy representations of the phenomena recorded ? If not, 

 what is the extent of truth, and where are the inlets for errors 

 and mistakes ? Not only has photographic observation gained a 

 wide range of applications in astionomy, but in every other 

 branch of physical science its help is daily more and more taken 

 advantage of; and alihough, in speaking of this art, I shall con- 

 fine my,- elf to astronomy, the observations which I propose to 

 make may be suggestive with reference to other branches of 

 physics. 



As an instance of the application of this art to optical physics, 

 1 may in this place call attention to the very successful delinea- 

 tion of the solar spectrum by Mr. Lewis M. Rutherfurd, of the 

 United States. In Mr. Rutherfurd's spectrum, obtained by the 

 camera, many portions and lines are shown (in the ultra-violet 

 for instance), which, while imperceptible to the retina of the eye, 

 impress themselves on the sensitive film. As a fact, lines which 

 sie sirgle in Angstibm's .and Kirchhoffsmaps have been recorded 

 by photography as well marked double lines. I will now review 

 the application of the art to astronomy. 



Stellar photography was for some time applied at Harvard 

 College Obsen-a.ory, U.S., to double stars, for the purpose of 



determining by micrometric measurement their relative angle of 

 position and distance. The zero of the angle of position was 

 found by moving the telescope in right ascension after an im- 

 pression had been taken, and taking a second one on the same 

 plate ; this process gave two sets of photographic images on the 

 same plate, and the right line passing through the series gave the 

 direction of the daily mo'ion of the heavens. The probable 

 error of a single measurement of the photographic distance of 

 the images was found to be ±o"'i2, or somewhat smaller than 

 that of a direct measurement with the common filar micrometer. 

 The late Prof. Bond, who applied photography to stellar 

 astronomy, confining himself to stars brighter than the seventh 

 magnitude, discussed the results in various numbers of the 

 '* Astronomische Nachrichten. " No astronomer more unbiassed 

 could have been selected to decide on the comparative value of 

 the photographic and direct observational method. His dis- 

 cussion shows that the probable error of the centre of an image 

 was ±o"'o5l, and that of the distance of two such centres was 

 ±o"'072. Adopting the estimate of Struve, ±o"'2i7, as the 

 probable error of a single measurement of a double star of tliis 

 class with a filar micrometer. Prof. Bond shows that the 

 measurement of the photographic images would have a relative 

 value three times as great. He derived the further important 

 conclusion, that deficiency of light can be more than compen- 

 sated for by proportionate increase in the time of exposure. A 

 star of the ninth magnitude would give a photographic image, 

 after an exposure of ten minutes, with the Cambridge equatorial. 



In the reproduction of stars by photography, recently under- 

 taken by Mr. Rutherfurd, the objects to be secured being sj 

 minute, special precautions were found to be necessary in de- 

 picting them upon the sensitive film, so that their impressions 

 might be distinguishable from accidental specks in the collodion 

 plate. To prevent any such chance of mistake, Mr. Rutherfurd 

 secures a double image of each luminary, the motion of the 

 telescope being stopped for a short time (hilf a minute) between 

 a first and second exposure of the plate ; so tliat each star is 

 represented by two close specks, so to speak, upon the negative, 

 and is clearly to be distinguished by this contrivance from any 

 accidental speck in the film. A map of the heavens is thus 

 secured, very clear though delicate in its nature, but yet one 

 upon which implicit reliance can be placed for the purposes of 

 measurement. Prof. Peirce aptly says, "This addition to 

 astronomical research is unsurpassed by any step of the kind 

 that has ever been taken. The photographs afford just as good 

 an opportunity for new and original investigation of the relative 

 position of near stars as could be derived from the stars them- 

 selves as seen through the most powerful telescopes. They are 

 indisputable facts, unbiassed by personal defects of okservation, 

 and which convey to all future times the actual places of the 

 stars when the photographs were taken." 



Mr. Asaph Hall, who shared with Prof. Bond the work ot 

 measuring the photographic images, and of reducing the mea- 

 surements, has very recently subjected the photographic method 

 to a critical comparison, with a view to deciding on its value 

 when applied to the observation of the transit of Venus. He 

 appears, as regards its application to stellar observations, to 

 uiider-estimate the photographic method in consequence of want 

 of rapidity ; but he admits that in the case of a solar eclipse, or 

 of the transit of a planet over the sun's disc, it has very great 

 advantages, especially over eye-observ.itions of contacts, inner 

 and outer, of the planet and the sun's limb, and that the errors 

 to which it is subject are worthy of the most thorough investiga- 

 tion. The observation of a contact is uncertain on account of 

 irradiation, and is also only momentary : so that, if missed from 

 any cause, the record of the event is irretriev.»bly lost at a par- 

 ticular station, and long and costly preparations rendered futile. 

 On the other hand, when the sky is clear, a photographic image 

 can be obtained m an instant, and repeated throughout the pro- 

 gress of the transit, and even if all the cont.icts be lo-t, equally 

 valuable results will be secured, if the data collected on the 

 photographic plates can be correctly reduced, as will be proved 

 hereafter to be undoubtedly possible. -That the transit of Venus 

 will be recorded by photography may now be announced as cer- 

 tain, as preparations are energetically progressing in England, 

 France, I'ussia, and .America for obtaining photogiaphic records. 

 There is also a posibility of Portugal taking part in these obser- 

 vations ; for it is contemplated by Sei"ror Capello to trans-port the 

 Lisbon photoheliograph to Macao. There are at present five 

 photoheliographs in proce.=s of construction for the observing 

 parlies to be sent out by the British Government, under the 



