July, 1902.] 



KNOWLEDGE 



157 



of tlie Swan is shown in Fig. 13. The Cross in the Swan 

 is to bo seen about overhead when darkness is coming 

 on during the months of July, August, and September. 

 Fig. 14 shows /? Cygni as s«en by me at about 

 Sh. 46m. p.m.. Aug. 21, 1895. The companion-star of 

 /3 is a lovely blue colour. For the purpose of observing 

 star-gi-oups it is sometimes desirable to use the terrestrial 



* ♦: 



Cassiopeia's ■ 

 Chair *. I 



} 



+ «, 



*a 



Great ' 



Cluster in ', ^ 



Perseus 





*i^ 



+„ I Chief Stars in Andromeda 



■ ^r 



Fio. 15. 



ej'e-piece without the two lenses nearest to the object 

 glass ; that is to say, without the erecting lenses. The 

 two lenses nearest the eye-end are to be left in theii- 

 places. After replacing the eye-])iece when the first-- 

 mentioned lenses have been removed, the draw-tubes of 

 the telescope will have to be pushed in much further 

 than Ix-fore in order to get the telescope into focus. 

 By removing the two lenses a large field of view is 

 obtained, but the magnifying power is much lowered, 

 so this method should not be resorted to when observing 

 planets. 



Fig. 15 — Cassiopeia's Chair, Chief stai-s in Andro- 

 meda. Great clust«r in Perseus. Cassiopeia's Chair is 

 on one side of the Pole-star; Charles's Wain on the 

 other. There arc veiy fine fields of stars to be seen 

 in this region in the Milky Way. The cluster in Perseus 

 is a fine sight, especially with a low power on a H or 

 2-inch telescope. 



Fig. 15 shows the chief stars of Andromeda as situated 



Fio. 10. 



with regard to Cassiopeia's Chair. The Nebula will bo 

 found by moving the telescope backwai-ds and fonvards 

 horizontally in the neighboiuhood of the star v. At 

 the end of each horizontiil sweep of tho instrument, 

 elevate it a little, and then move it back again, only 

 over a higher strip of sky parallel to, and partly over- 



lapping the first strip. Repeat this process till the 

 Nebula comes into view. Yoxi must, of coui'se, make 

 ruro of beginning your search hrlow the position of tho 

 Nebula. By making the strips of sky overlap the chance 

 of missing the Nebula between tho swcx;ps of the 

 telescope is avoided. The Nebula itself is oval, brightest 

 at the centre. This is a sketch of the Nebula made by 

 mo at about 5h. 35m. p.m., Saturday, Nov. 16, 1895. 

 My l|-inch telescope minus the two louses nearest the 

 object glass was used in making this sketch. The 

 Nebula appeai-s to consist of concentric rings as seen in 

 a photograph of it taken by Dr. Isaac Roberts in 

 Dec. 1888. 



The star y Andromedse is a double star, the companion 

 being sea-green. The companion itself was found to be 

 double by Otto Stanivo in the year 1842. 



In bringing these notes to an end, I hope I have 

 given sufficient information to induce some to begin to 

 study tho wonders of the heavens, oven though the 

 iustnunonts at their command may be of so humble a 

 description as a mere hand telescope. 



Utttcrs. 



[The Editors do not. hold themselTcs responsible for the oinnions 

 or statements of correspondents.] 



THE VISIBILITY OF THE CRESCENT OF VENUS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Before ■« e can accept as facts such prodigies of 

 vision as the detection of the crescent of Venus, or of the 

 elongated figure of Saturn, with the unaided eye, we ought 

 to be able to exclude one source of error which is 

 very likely to vitiate such observations, viz., the regular 

 astigmatism of the eye which is present in almost all 

 cases. 



The very name of this optical defect serves to indicate 

 the way iu which it may lead to mistakes. It was coined 

 by Whewell iu order to emphasize the fact that rays of 

 light proceeding from a point do not again unite iu a 

 point after passing through a refractive system of the 

 particular kiud called astigmatic. 



In the human eye the cause of astigmatism is the 

 asymmetry of the cornea, and the direction iu which the 

 elongation of a point into a line will take place will depend 

 upon the direction in which the planes of greatest and 

 least refractive power are situated. It is most common 

 to find that the cornea is more strongly curved in the 

 vertical direction, and less strongly in the horizontal, but 

 the rule is not invariable. 



If then anyone thinks that he can detect any " elonga- 

 tion " of Venus or of Saturn with the unaided eye, he 

 ought to check his observation, before rushing into print, 

 in the following way, repeating the observation with each 

 eye separately. Let him notice the exact direction in 

 which the elongation is seen. Then, keeping his eye on 

 the star, let him bend his head gradually down toward 

 one side, and let him notice carefully whether the direction 

 of the apparent elongation changes as he does so. When 

 the head is (piite down on one side the positions of the 

 two planes of greatest and least refraction resi.>ectively will 

 have changed places, since iu regular astigmati.sm they 

 are at right angles to one another in the eye. AccoidiiiLjly 

 the direction in which the star appears lengthened will be 

 now seen to be at right angles to that of its original lengthen- 

 ing if the elongation is due to ocular astigmatism, and will 



