252 



KNOWLEDGE. 



July, 1913. 



Saturn is a morning star, coming into a better position for 

 observation. Polar semi-diameter 8l". P. is — 4°-7; ring 

 major axis 41. i", minor 1 8 J". The ring is very widely open. 

 It is of interest to examine the exact amount of overlap 

 beyond the planet's pole. 



East Elongations of Tethys (every fourth given), 4 d 

 ll h -7m, 12" l h -0m, 19" 2 h -3e, 27"3 h -6m; Dione 

 (ever third given), 5 d 2 h • 5m, 13" 7 h • 7m, 21" h • 8e, 

 29" 6 h ■ Oe ; Rhea (every second given), 6" l h • 7m, 15" 2 h • 8m, 

 24" 3 h • 8m. For Titan and Iapetus E.W. mean East and 

 West Elongations, I. Inferior (North) Conjunctions, S. 

 Superior (South) ones. Titan, 2" 5 h • 7e W., 6" 5 h ■ 3e S. ; 

 10" 8 h • 9e E., 14" 9 h • 4e I., 18" 6 h ■ Oe W., 2" 5 h • 6e S., 



26" 9 h • le E., 30" 9 h • U I. ; Iapetus, 20" 8 h • be W. 



Uranus was in opposition on July 29th. Semi-diameter, 

 lj". At end of August, 2° S.E. of p Capricorni. 



Neptune is too near the Sun for observation, having 

 been in conjunction on July 19th. 



Double Stars and Clusters. — The tables of these given 

 last year are again available, and readers are referred to the 

 corresponding month of last year. 



Variable Stars. — Tables of these will be given each 

 month ; the range of R.A. will be made four hours, of which two 

 hours will overlap with the following one. Thus the present 

 list includes R.A. 18 h to 22 h , next month 20 h to h , and so on. 



CORRESPONDENCE. 



Note. — We have received a communication from Dunford Bridge relating to Professor Thomson's note on 

 reproductive harmony in the wild duck (see "Knowledge," Volume XXXVI, number 538, page 187), and one 

 from "Old Planter," but as the writers have forgotten to attach their names, according to our rule, we are not 



able to print the letters. 



ASTRONOMICAL PHOTOGRAPHY. 



To the Editors of " Knowledge." 



Sirs, — Would you be so kind as to give me some hints 

 about astronomical photography. I have an ordinary quarter- 

 plate hand-camera, and a 2i" refracting telescope. I should 

 be extremely grateful for any advice on the subject. 



W. P. WILLIAMS. 

 Arlington Park College, 

 Chiswick. 



In answer to the above letter, Mr. J. Milton Offord, F.R.M.S., 

 has kindly sent the following note. 



SIMPLE DIRECTIONS FOR PHOTOGRAPHY WITH 

 A SMALL TELESCOPE.— Photography of the Sun may be 

 taken with a very small telescope, using an enlarging lens in 

 place of the eyepiece and a screen close over the plate. The 

 exposures are so short that a fixed telescope will answer. 



For the Moon use the fastest plates obtainable with an 

 enlarging lens. Exposures of about one second at full aperture 

 can be given and the amount of enlargement possible is 

 dependent on the aperture of the telescope. Images about 

 one-inch diameter with a two-inch telescope should show a 

 good deal of detail. 



Planetary photography is beyond the reach of a small 

 telescope, except in the case of Venus, where the crescent 

 form can be photographed with an enlarging lens. 



For stellar photographs use the telescope as a guide to an 

 ordinary camera firmly attached to it. With patience, good 

 photographs may be taken by keeping a bright star in the 

 centre of the field of the guiding telescope. This is best done 

 by bisecting the out-of-focus image of the star with cross wires 

 in the eyepiece. Even a simple pillar and claw stand may be 

 used, but it is essential to balance well the telescope and 

 secure a comfortable rest for the observer's head and arm. 



The apparatus needed is quite simple: a light-tight 

 tapering box, securely fastened to the outside of the eye end 

 of the telescope leaving the rack tube free ; a focusing glass, 

 finely ground, with its centre rendered transparent by a 

 microscopic cover glass being cemented to it with Canada 

 balsam ; a plate-holder as light as possible ; a simple 

 magnifying glass in a screw mount for focusing, and a 

 dark card to cover the object-glass end of the telescope 

 will be required. The enlarging lens is important; it takes 

 the place of the ordinary eyepiece, and the best form is a 

 triple-cemented lens on Steinheil's principle, as used for 

 dissecting microscopes; cost about 10/-. This kind of lens 

 passes a great deal of light and gives sharp images. 



To take a photograph, say, of the moon : First obtain the 

 actinic focus, if a refracting telescope, by trial. To do this 



mark the draw-tube roughly in fractions of an inch, say for half 

 an inch, round the visual focus ; then make a number of 

 exposures at different focal lengths until the best result is 

 obtained. When this is found, the focusing lens can be adjusted 

 to view the moon through the central clear space in the ground 

 glass, and with the telescope at best actinic focus, made to give 

 good visual focus and fixed there, so that ever after when focusing 

 with this lens it will be the true focus for photography. 



Having obtained the focus, cover the object glass with the cap. 

 Set the telescope in advance of the Moon and wait until the 

 Moon's image will be central, having the plate open, and then 

 expose by removing and replacing the card. It is best to have 

 the card larger than the aperture and hold it in front for a while 

 before exposing, to prevent shaking the telescope, quickly 

 moving it to one side and back again to expose. 



For developers Metol Hydrokinone or Paramidophenol are 

 good. For plates, Imperial Flashlight or Lumiere Sigma 

 answer well ; they should, of course, be backed. 



THE "FOURTH DIMENSION." 

 To the Editors of " Knowledge." 



Sirs, — There is one statement in Mr. Redgrove's letter in 

 the June number of " Knowledge "to which I ought to reply 

 He says that he had " already intimated " that he did not 

 intend to discuss the question further with me. This is not 

 correct ; what he said does not bear this meaning. However, 

 I have no wish to continue the correspondence, and, since he 

 does not wish to reply to my letter in the April number of 

 " Knowledge," I am quite content that what is contained in 

 that letter should remain the final statement of the issue 

 between us. JOHN JOHNSTON. 



Hendon, N.W. 



THE PLANET MARS. 



To the Editors of " Knowledge." 



Sirs, — A copy of your Journal for May, 1913, has been sent 

 me, apparently to call my attention to, and perhaps secure my 

 comment upon, an article denying the reality of the canals of 

 Mars. Comment on the argument is unnecessary, but the 

 statement on the first page, that Schiaparelli is responsible for 

 the theory of their artificiality, should be corrected. The 

 blame for the discovery rests wholly on me. His theory was 

 that they were natural channels, though, with the magnanimity 

 and open-mindedness of genius, he wrote to me before his 

 death : " Votre theorie devient de plus en plus probable." 



PERCIVAL LOWELL. 



Lowell Observatory, 

 Flagstaff, A.T. 



