50 



ASTRONOMICAL PHENOMENA AND PEOGKESS. 



surprised to find that Venus, instead of being 

 about one-sixth of the diameter of the moon, 

 was rather more than double its diameter, so 

 that, when the adjustment was made to bring 

 the upper edge of the moon coincident with 

 the upper point of the crescent of Venus, the 

 opposite edge of the moon fell short of the 

 middle of the crescent, a very palpable demon- 

 stration of the fallacy of guesses at size, when 

 there are no means of comparison. 



" On another occasion a lady was looking at 

 Jupiter through my telescope, and having first 

 put on a power of 60 I changed it for one of 

 140. To. my question, what difference she ob- 

 served in the size of the planet, she answered, 

 1 1 see no difference in size, but a good deal in 

 brightness.' Here the area of the one image 

 was more than five times that of the other. 



" The fallacy of guesses at size without ob- 

 jects of comparison is most strikingly shown 

 in the ordinary expression of an ignorant ob- 

 server looking at objects by day through a 

 spy-glass. If you ask, as I have often done, a 

 person unacquainted with optics whether he 

 recognizes any difference in size between an 

 object, say a horse or a cow, seen with or 

 without a telescope, he will always answer, 

 ' No, but it ' (the telescope) ' brings it much 

 nearer.' This, of course, is really an admis- 

 sion of increased magnitude, but the observer 

 is unconscious of it ; a horse to him is as big 

 as a horse, no larger or smaller, whatever be 

 the distance. 



"Suppose all objects had never been seen 

 but at one and the same distance, then an ob- 

 server, looking at a given object without any 

 external standard of comparison, would prob- 

 ably make a fair guess at its size, for the pic- 

 ture on his retina would have a definite size, 

 and his mind would estimate it by relation to 

 other pictures of known objects which he had 

 seen at other times ; but, as we see all the ob- 

 jects with which we are familiar at all degrees 

 of distance, we have no standard of compari- 

 son for an image on the retina. 



" The common phantasmagoria effect, where 

 a figure appears to advance or recede from us, 

 though it really does not change its position, 

 but its size, is one of the many illusions pro- 

 duced by representing things as they are seen 

 under certain circumstances which have be- 

 come habitual, and habit interprets the vision. 

 So if one lie on his back in a field, and, throw- 

 ing the head back, look at distant trees or 

 .houses, they will appear to be in the zenith, 

 because when we ordinarily look at the zenith 

 the head is thrown back. 



"Is the apparent size of the sun or moon, 

 as expressed in common parlance, any thing 

 more than a reference to some standard which 

 we have early adopted, and which, not having 

 any means of rectifying, we assume? To me 

 the moon at an altitude of 45 is about six 

 inches in diameter; when near the horizon, 

 she is about a foot. If I look through a tele- 

 scope of small magnifying power (say 10 or 12 



diameters), so as to leave a fair margin in the 

 field, the moon is still six inches in diameter, 

 though her visible area has really increased a 

 hundred-fold. 



" Can we go further than to say, as has often 

 been said, that all magnitude is relative, and 

 that nothing is great or small except by com- 

 parison ? " 



The Cordova Observatory. The Congress of 

 the Argentine Republic having voted to estab- 

 lish a national observatory at Cordova, Dr. B. 

 N. Gould, of Cambridge, was invited to super- 

 intend its erection and take charge of it. He 

 accepted the offer, and left this country for 

 Cordova during the past summer, accompanied 

 by a number of experienced assistants. Dr. 

 Gould will there have an opportunity to extend 

 the catalogue of the southern heavens beyond 

 the limit of 80, to which the zones of Arge- 

 lander extend. The two largest instruments 

 will be a Repsold meridian circle of 54 inches 

 focal length, and 4J- in. aperture, and an equa- 

 torial by Alvan Clark & Sons, provided Avith 

 the 11-inch object-glass lately in possession of 

 Mr. Rutherfurd. The Coast Survey, the Smith- 

 sonian Institution, the American Academy of 

 Arts and Sciences, the Washington Observa- 

 tory, and Nautical Almanac Bureau, as well as 

 astronomers in England, Germany, and Russia, 

 have taken much interest in the enterprise, and 

 contributed valuable instruments and books. 

 Dr. Gould writes of his plans as follows to the 

 American Journal of Science for July : 



" My hope and aim are to begin a few degrees 

 north of Argelander's southern limit, say at 

 20 or 27, and to carry southward a system of 

 zone-observations to some decimation beyond 

 Gilliss's northern limit, thus rendering com- 

 parisons easy with both these other labors, and 

 permitting the easy determination of the cor- 

 rections needful for reducing positions of any 

 one of the three series to corresponding ones 

 for the other. It is, of course, impossible to 

 arrange in advance the details of such an un- 

 dertaking, but my expectation is to go over the 

 region in question in zones 2 wide (except in 

 the vicinity of the Milky "Way, where the width 

 would be but one-half as great), up to a decli- 

 nation of about 55, after which the width 

 would be gradually increased as the declina- 

 tions became greater. Within these zones all 

 stars seen as bright as the 9th magnitude would 

 be observed, so far as possible, moving the 

 telescope in altitude when no bright star is in 

 the field until some one becomes visible, ac- 

 cording to the well-known method of zone- 

 observations. 



" For reducing the observations, differential 

 methods will probably be employed, inasmuch 

 as the time now assigned for my absence from 

 home would be inadequate for proper discus- 

 sion of the corrections* required for nice deter- 

 minations of an absolute character. Still it is 

 my present purpose, as far as possible, to make 

 such subsidiary determinations as might here- 

 after be needed in any attempt at computing 



