May, igo6 ] 



KNOWLEDGE & SCIENTIFIC NEWS. 



421 



gradually and without shock, which would not be the 

 case if the pendulums were rigidly joined to the rest of 

 the driving mechanism. 



A larger clock controlled by five pendulums, each 

 3 feet 3 inches long, was subsequently used, and when 

 everything was properly adjusted the motion was so 

 reliable and certain that when engaged in photographic 

 work it was only necessary to examine the image in the 

 guiding telescope at intervals of a few minutes. Not- 

 withstanding extended trials with both types, it is diffi- 

 cult to express a decided opinion whether five pendu- 

 lums give better results than three. A greater measure 

 of steadiness appears to be secured by adding to their 

 length than by increasing the number. Three must, 

 however, as already pointed out, be regarded as the 



Fig. 6. 



minimum, and their length should preferably not be 

 less than three feet. 



Having briefly described the various appliances which 

 may be brought into u.-^e by those desirous of taking up 

 astronomical photography, a few hints, biused on the 

 writer's experience, will now be gi\en regarding 

 methods of working. 



Solar Photography. — In selecting plates for photo- 

 graphing the solar image the slowest brand procurable 

 should be adopted; indeed, it may be pointed out as an 

 invariable rule, that, where a sufficiently long exposure 

 can be given with a slow plate, it should always be 

 selected in preference to one of greater rapidity. Rapid 

 plates do not give the same amount of delicate detail, 

 neither do they admit of the same latitude in exposure. 

 It is only in ca.ses where very prolonged expn^sures ai'e 

 ejisential that their u.se becomes a necessity, at least so 

 far as astronomical purposes are concerned. I-'or solar 

 work a good brand of lantern plate will probably be 

 found to give the best all-round results. All piates 

 should be backed with a suitable medium. No hard- 

 and-fast rule can he laid down with regard to the dura- 

 tion of exposure, as this is appreciably affected by the 

 position of the sun at the time the photograph is 

 secured, but as a rough guide it may be slated that on 

 a summer day w ith the sun placed in a clear skv, and the 



telescof>e stopped down to an angular aperture of f /60, 

 an exposure of i-40th of a second on a lantern plate 

 will be found to be ample. In other words, if a tele- 

 scope of a focal length of 10 feet is stopped down to an 

 effective aperture of two inches, the exposure required 

 will be about i-4oth of a second. If the primary image 

 is enlarged by a lens, a correspondingly increased ex- 

 posure must, of course, be given. 



Lunar Photography. — In taking photographs of the 

 moon, the beginner will soon discover that, in order to 

 secure the desired amount of detail, the terminator 

 requires a longer exposure than the rest of the illu- 

 minated disc, and that by giving exposures of varying 

 duration the apparent age of the moon may be altered 

 by several hours. At least three methods may be 

 brought into use for getting over this difficulty. (l) 

 A strip of cardboard, one end of which is similar in size 

 and shape to the lunar terminator, may be introduced 

 by the hand into the end of the telescope tube, and 

 moved slightly backwards and forwards in front of the 

 shutter for, say, two seconds, and rapidly withdrawn. 

 It is to be understood that during the first part of the 

 exposure the terminator only is to remain uncovered, 

 and, if the shutter is closed after an interval of another 

 second, the terminator will thus get an exposure of 

 three seconds, and the rest of the disc one-third of that 

 amount. (2) The plate may be given an exposure in the 

 telescope suited to the terminator, and afterwards de- 

 veloped. A positive is then taken from the negative by 

 contact, but during the exposure of the plate to the 

 source of illumination a strip of cardboard, similar to 

 that prexiously referred to, is brought into use, and 

 moved slightly backwards and forwards over the image 

 of the terminator during two-thirds of the total ex- 

 posure, and then withdrawn. Another negative can 

 be taken from the positive thus obtained either by con- 

 tact, or, if a camera is used, an enlarged image may 

 be obtained at the same time. (3) The plate may be 

 exposed in the telescope as before, and the resulting 

 negative afterwards reduced in density where necessary 

 by the local application with a soft brush of a reducing 

 solution, such as that known as " Farmers " (Satu- 

 rated solution of Ferricyanide of Potassium i part. 

 Hyposulphite of Soda solution i to 5, 10 parts). If 

 the method first referred to should not give the result 

 aimed at, the defect may be removed by method No. 2 

 or 3, or all may be partly utilised in the production of a 

 single negative. 



If a lens is used in the eye-tube of telescope to en- 

 large a portion of the primary image, the illumination 

 will be more evenly distributed, and the plate under 

 such circumstances may be treated in the usual way. 



Planetary Photography. — Unless a telescope of long 

 focus is avaihible, no very satisfactory photographs oi 

 any of the planets can be obtained, and even the best 

 results, which ba\e so far been secured compare un- 

 favourably with the \ lews which a comparativelv small 

 instrument presents to the eye of the observer. When 

 dealing with such a small object, it is advisable to use 

 an eye-piece in conjunction with the telescope to enlarge 

 the primary image, and in the case of the brighter 

 planets the aperture should be stopped down to reduce 

 the intensity of the light with the view of being better 

 able to regulate the exposure and development. Care 

 should be exercised that these are not too prolonged, 

 otherwise the small discs will be enlarged by the effect 

 of halation, and all detail effectuallv ob.scured. 



On the accompanying plate a few examples are given 

 if planetary and lunar photography. With the excep- 

 tion ot l-'ig. 10, where a lo^ inch mirror of 6 feet 



