2 78 



NA TURE 



[January 17, 1901 



use, which impart increased precision to the results. 

 Michel-Levy's elaborate discussion of the optical proper- 

 ties of the felspars has greatly facilitated the discrimina- 

 tion of the several varieties ; while among special 

 instruments we may recall Fedorow's " universal theodo- 

 lite," enabling accurate optical measurements to be made 

 on crystals in random sections. The application of dense 

 "liquids to determine the specific gravity of minute frag- 

 ments of minerals, and to obtain pure material for 

 analysis, has been perfected, and, especially in the form 

 of Sollas's "diffusion column," has been made both 

 simple and convenient for use. The importance of 

 accuracy and completeness in the chemical analysis of 

 rocks has become more generally recognised. Many of 

 the most valuable rock- analyses published in late years 

 are due to the United States Geological Survey, who 

 have realised the importance of maintaining an adequate 

 staff of skilled chemists. One point brought out is the 

 wide distribution in igneous rocks of small amounts of 

 the heavy metals, and of some other elements, such as 

 barium and strontium. Apart from its obvious applica- 

 tion to questions concerning the origin of metalliferous 

 deposits, information of this kind will probably be found 

 to throw some light upon matters of more strictly 

 petrological interest. 



THE DISAPPEARANCE OF IMAGES 

 PHOTOGRAPHIC PLATES. 



ON 



T T is the aim of the modern astronomer to employ 

 -'• photography, whenever possible, in the many 

 • branches of his work, as by this means the peculiarities 

 of the observer are eliminated and a permanent record 

 is obtained that can be exammed at leisure at any later 

 date. In some kinds of work photography helps us in 

 obtaining a great number of facts in a very short space 

 of time, facts which would have taken weeks to accumu- 

 late by the old method of eye observation. Not only is 

 the science more rapidly advanced by the greater abun- 

 dance of material at hand, and therefore available for 

 discussion, but the application of photography to astro- 

 nomy has opened up so many new fields of work that 

 the whole subject has now a far wider horizon than 

 before. 



To be able to photograph in a few hours objects 

 which for ever will be outside the reach of the human 

 eye, however aided, is one of the many marvels of this 

 invaluable method, 



The fine photographs obtained to-day, and so well 

 illustrated in our books by the aid of the modern processes 

 of reproduction, suggest the importance of recording 

 the appearances of these celestial objects after some 

 years have elapsed, in order to be able to note whether 

 any changes in brightness or form are occurring. The 

 magnificent work of the " Carte du Ciel," started by the 

 far-seeing mind of Admiral Mouchez, is one of those 

 schemes on a large scale for obtaining a survey of the 

 universe at the close of the nineteenth century. The 

 many thousands and thousands of photographic plates 

 that will have been exposed to the sky when this plan will 

 have been accompHshed point out to us the immense 

 importance that must be attached, not only to the 

 *' keeping " qualities of the film on the glass plates, but 

 also to the retention of the images on these films. 



Since one of the chief objects of Admiral Mouchez's 

 plan was to hand down to posterity a chart of the celes- 

 tial vault as recorded in our time, to enable those that 

 follow us to compare with it that recorded in their time, 

 the importance of the preservation of the photographic 

 films, and the images impressed on them, cannot be over- 

 rated. 



Most of the readers of Nature are familiar with those 

 beautiful photographs of long exposure of nebulas and star 



NO. 1629, VOL. 63] 



clusters which we owe to the skill and patience of Dr. 

 Isaac Roberts, and which he has given to the world in 

 two magnificent volumes of plates. In the second of 

 these volumes Dr. Roberts makes some very interesting 

 and valuable remarks regarding the "fading" of some 

 of the images on the photographic plates, and his ex- 

 perience shows that such disappearances of images after 

 long intervals of time may assume very considerable 

 proportions. 



Although the period over which his experience extended 

 only covered about ten years, this interval of time was 

 sufficiently long to enable him to obtain some very striking 

 facts. 



The following brief summary of the two instances 

 which he recorded iii his second volume will give the 

 reader a general quantitative idea of the disappearances 

 of images during a short period. 



A photograph of a region of the sky was taken on 

 February 15 in 1886, and 403 star-images were counted 

 on the negative. A re-count on May 29, 1895, found only 

 272 images, a loss of 131 images in about nine and three- 

 quarter years. 



Again, another photograph of identically the same 

 region, taken on March 22, 1886, recorded 364 stars ; the 

 same plate, examined in May 1895, showed only 234. 



These facts, then, indicate that, even after so short 

 a period as ten years or so, the photographic film cannot 

 be depended on, and that for lasting purposes recourse 

 must be made to some means of reproducing the photo- 

 graphs soon after they have been obtained, the reproduc- 

 tions being as faithful to the originals as possible. It is 

 generally known that, in nearly every method of repro- 

 duction, very faint details are lost in the process, but 

 this loss could be easily recorded by noting the slight 

 differences at the time ; printed on good paper and with 

 permanent ink, such reproductions should be lasting. 



The fact that the great photographic chart of the sky 

 is approaching completion causes one to think of the 

 great expense that will be involved in reproducing the 

 large number of individual plates. That many of the 

 observatories which have taken part in this undertaking 

 will find some difficulty in at once getting together the 

 necessary funds is quite possible, so that a delay of a 

 few years may be detrimental to the accuracy of the 

 chart. 



It is therefore very important that some means should 

 be at hand to prevent the images of the fainter objects 

 from fading quite away, or, if already disappeared, to 

 bring them back to view. 



Fortunately, the latter alternative can be accomplished 

 by a process of manipulation which we owe to Sir 

 William Crookes. 



The completeness and success of this method will be 

 gathered from the fact that when Sir William Crookes 

 had "revived" one of Dr. Roberts' negatives that had 

 entirely lost many images, the latter, on re-counting, 

 " found that every one of the missing images had been 

 restored to view, as distinctly, I think, as they were after 

 the negatives were first developed." 



The process by which Sir William Crookes accom- 

 plished this has been published in detail by Dr. Roberts 

 in the Monthly Notices of the Royal Astronomical Society 

 (vol. Ixi., No. I") for last November. As the description 

 should prove of interest and service to many of our 

 readers, the following account is appended, as described 

 by Sir William Crookes himself in a letter to Dr. 

 Roberts. 



"(i) Soak the plate for three hours in distilled water. 

 (2) Prepare, in advance, two solutions A and B. 



Solution A. 



PyrogalHc acid i oz. 



Sodium metabisulphite I oz. 



Water 80 oz. 



Solution B. 

 Sodium carbonate 



(crystals)..... 12 oz. 



Sodium sulphite 4 oz. 



Water 80 oz. 



