Mat 26, 1882.] 



• KNOWLEDGE 



621 



planet Uranus the introduction of the November meteor 

 zone into our solar system, either through a close approach 

 of that giant planet to the meteor family at that point 

 of its track where it still approaches the path of Uranus, 

 or else from the actual expulsion of meteoric fragments 

 from the interior of Uranus millions of ages ago, when the 

 planet was passing through its sun-like stage. Be this 

 as it may, it is certain that the November meteor system — 

 tJiough at the verj' time when these words were written 

 it lay in the same direction from the sun as our earth 

 does — yet lies some nineteen times as far away, or, roughly, 

 alx)ut 1,6.50 millions of miles further from the sun than we 

 are at this present time. To charge the November meteor 

 system, therefore, with robbing our earth of a portion of 

 its supplies of solar heat is to i i.itate the wolf in the fable, 

 who accused the lamb of troubling the stream, though the 

 stream llowed from the wolf towards the lamb. The sun's 

 rays in May pass our earth on their way to the November 

 meteor system, not that system on their way to the earth. 

 It is, therefore, now a matter of scientific certainty that 

 the cold snap in May is not caused by the November 

 meteor system. As for the August meteors, their track 

 has been found to be even far wider, and in February, 

 when Ertel supposed they were intercepting our supplies 

 f'f solar heat, they lie forty times farther from the sun 

 tlian our earth. 



It does not of necessity follow that Ertel's theory is 

 erroneous ; Viut now that tlie evidence he seemed to find in 

 its favour from the November and August meteors has 

 been shown to be fallacious, the defects which from the 

 first characterised the theory will be more readily admitted. 

 These are sufficiently manifest If meteors diminished 

 our supply of heat, it could only be by coming actually 

 L between the earth and the sun in such a way that they 

 could be actually visible (if sufficiently magnified with a 

 telescope) upon the sun's face. If they formed a fine 

 cloud of cosmical dust, they would reduce his apparent 

 brightness in a measurable degree if they affected in such 

 degree the supply of heat we received from him : and this 

 has never been noticed. If they were individually large 

 enough to reduce the supply of heat, they ought to be indi- 

 vidually visible as black objects crossing his face even to 

 the naked eye, but certainly Avith powerful telescopes. No 

 such bodies have ever been seen. 



However, there is a terrestrial test for the theory by 

 which its validity could be readily determined. If meteoric 

 bodies come between the earth and the sun at any time in 

 such numbers as to make us feel cold in their shadow, they 

 cool the whole earth, not England, or Europe, or the 

 northern hemisphere. If, then, on a careful comparison 

 of the mean daily temperature at observatories all over the 

 world, it is found that the cold snaps of February, April, 

 and May are everywhere to be recognised, then it must 

 be admitted, at least, that the cause of the peculiarity 

 is to be sought outside the earth herself. If, on the 

 contrary — as, for my own part, I suspect would prove 

 to be the case — no trace of these cold snaps could be 

 recognised — say, in Australia and South Africa, though 

 cold snaps at other dates might be noticed — then the pecu- 

 liarity must be regarded as local It may extend over 

 the British Isles, or even the whole of Europe, or it may 

 even, though this seems unlikely', prevail over much 

 larger portions of the northern hemisphere ; but if not 

 recognised (and that, too, at precisely the same epochs) in 

 both hemispheres, its cause cannot be regarded as extra- 

 terrestrial. It should not be difficult, if the cause is on 

 our earth, to trace it to its source. I should not be sur- 

 prised if that source were found to lie among the ice- 

 fields of the North Atlantic. 



PHOTOGRAPHY FOR AMATEURS. 



By a. Brothers, F.R.AS. 



PART VIII. 



LANDSCAPE photography, portraiture, copying, and 

 various other branches of photography may be fol- 

 lowed by the amateur after he has mastered the working 

 details given in previous papers. But there is one branch 

 to which many readers of Knowledge will naturally turn 

 their attention as soon as they can manipulate a collodion 

 plate. Astronomical photography is one of the most 

 fascinating sections of the art-science, and it is one in 

 which it is not difficult to achieve success, always sup- 

 posing that a good telescope is available. The method of 

 procedure is the same whether the telescope be of the re 

 fracting or reflecting form. With a reflecting telescope, 

 there is no difficulty with the actinic focus ; but with a 

 refractor, allowance has to be made in a way presently to 

 be referred to. We will describe the apparatus necessary to 

 attach the jilate-holder to a refractor. The eyepiece must 

 be removed, and in its place a piece of tubing of the same 

 diameter must be provided. To this tube a piece of flat 

 brass plate must be attached, and the dark slide of the 

 camera may be arranged so that it will slide on to the 

 brass plate and remain secure in any position of the tele- 

 scope. No camera is required — merely the means of hold- 

 ing the prepared plate. On a plate 4 j by 3|, two pictures 

 of the moon may be taken, merely by shifting the slide 

 about one diameter of the moon, after making the first 

 exposure. With a slightly-varied arrangement of the brass 

 plate and dark slide, four exposures can be made on the 

 same plate. W'ith telescopes up to 6 ft focal length, the 

 size of plate named will be sufficient ; but with larger in- 

 struments, a plate 5 by 4, or 6i by 4J, would be preferable. 

 The preparation of the plate is just the same as for a 

 landscape or copy. In order to find the focus, a piece of 

 ground glass may be placed in the dark slide, with the 

 ground surface towards the object, and the image of the 

 moon may be brought into sharp visual focus. The focus- 

 sing tube of the telescope having been drawn out so as to 

 allow some " play " for finding the actinic focus, a slight 

 scratch should be made on the tube to show the point of 

 visual focus, the sensitive plate may be put into the slide 

 and the shutter withdrawn. The cap of the telescope 

 should be removed, and a cover of some opaque material 

 substituted, and made to fit loosely, so that, in making the 

 exposure, the telescope should not be disturbed. The 

 driving clock having been adjusted to the moon's motion, 

 which can only be done approximately, the object glass 

 covered, and the prepared plate in its place, the 

 temporary cap may be carefully removed, and held a 

 short distance off', so as to allow any vibration of the 

 telescope to subside, and the exposure may then be made. 

 The time of exposure must be varied according to the 

 phase of the moon, the focal length of the telescope, and 

 the state of the atmosphere. At the time of full moon, and 

 if the sky be very clear, the time of exposure may be one 

 second or less — experiment alone will determine this point. 

 The first image taken in the visual focus is sure to be in 

 distinct ; and now, to find the actinic focus, turn the 

 focussing tube oiiticardg — say one-eighth of an inch — and 

 then take another picture, taking care that the slide has 

 been moved forward ; at the same time, a mark miist be 

 made on the brass tube. Proceed thus, carefully noting 

 the improved sharpness of the image. It is obvious that, 

 if the second picture is less distinct than the first, the 

 plate must be placed nearer to the object-glass. The focus 

 is longer or shorter, according as the object-glass is over or 

 under corrected. 



