I62 



NATURE 



December 15, 1892 



tinder consideration the size of the image thrown on the 

 screen can be varied at will by simply altering the 

 distance between the elements, but the further the lens is from 

 the focussing screen, the more will be the time of exposure. 



With such a lens as this Mr. Dallmeyer has taken many ex- 

 cellent pictures, but perhaps the best idea of its properties will 

 be gathered from the facts obtained by photographing— by means 

 of two cameras, one supplied with a "long focus landscape 

 lens," and the other with the "new telephotographic lens" — 

 the flame of an oil lamp placed at a distance of 20 feet. With 

 equal extensions of the camera the image of the flame given by 

 the new lens was five times greater than that by the other. 



In the compound lens the anterior element before referred to 

 is here replaced by a complete portrait lens, while a negative 

 symmetrical combination takes the place of the posterior 

 element. This lens may be said to be more perfect than the 

 siviple lens, Mr. Dallmeyer having been able to introduce con- 

 siderable improvement in the construction. 



Some excellent work done with this lens has been exhibited 

 by Messrs. F. Mackenzie and Annan at the Camera Club. The 

 pictures represented a building at a distance of 500 yards. The 

 first, taken with an ordinary rapid rectilinear lens with an exten- 

 sion of 14 inches, gave the house as | of an inch long. The 

 second — with the compound tele-photo lens, extension 9 inches 

 from the back lens — gave 2\ inches as the size of the house, while 

 the third, with 30 inches' extension, gave the house as 6g inches. 

 Although these numbers can give one a very good idea of what 

 this new lens can accomplish, yet the direct copies from photo- 

 graphs inserted in the pamphlet under consideration convey a 

 more vivid impression. 



There is no doubt that this lens will find some very valuable 

 applications, that of astronomical photography not being the 

 least of them, for every one knows the great advantage a short 

 telescope has over a long one if the degree of magnification in 

 both are equal. W. 



filled with rectilinear crystals of ice, which assume a curved 

 form in the mud at their margins. The peculiarity on Sunday 

 was their large size and beauty. Something analogous takes 

 place when gold or silver is reduced frr m solutions of its salts 

 by more electro-positive metals. Under certain circumstances 



w 



ARBORESCENT FROST PATTERNS. 

 E have received the following letters with regard to 

 arborescent frost patterns, to which Prof. Meldola called 

 attention in last week's Nature : — 



I AM very glad that Prof. Meldola has called attention to the 

 curved figures of frozen mud (of which the specimens on 

 December 4 were unusually fine), because I hope that some one 

 will explain why the sexangular crystallization which is universal 

 in snow, and general in water, is exchanged both on windows 

 and on muJdy pavements for curves. Probably I ought to know 

 all about it, but I cannot remember seeing an explanation, and 

 shall be obliged by reference to one, which will probably be of 

 interest to others besides G. J. Symons. 



62, Camden-square, London, N.W. 



The interesting "fronds" of muddy ice observed by Prof. 

 Meldola (p. 126) are not very uncommon on the pavements in 

 these " Northern Heights." I saw them on the date which he 

 named, and have more than once studied them. I then noticed 

 that the "interstitial " pavement seemed partly cleared of mud, as 

 ifthe water had drawn this towards the groups of crystals. The 

 mode of formation recalled to my mind certain phenomena in 

 crystal building within rocks, and I suspect the mud has its 

 influence. Indeed, it seems to me very probable that all these 

 " dendritic " growths of crystals are the results of " impeded " or 

 " constrained " crystallization, to some of which I have called 

 attention in noticing a structure in the Charnwood syenite 

 {Quart. Jour. Geol. Soc, 1891, p. loi). On this point 

 Prof. Sollas makes some important remarks in his well-known 

 paper on the Wicklow granites. T. G. BoNNEY. 



The beautiful curved forms assumed by the ice on the paving 

 flags last Sunday were very noticeable in this neighbourhood 

 and Hampstead as well as in other parts of London. What I 

 observed were not quite like those described and figured by 

 Prof. IMeldola, but resembled rather the scrolls and volutes 

 which are frequently used in decorative art. The finest piece 

 that I saw was in this square, where several of these scrolls 

 radiated from a central point, and spread over several feet of 

 the pavement. A friend, Mr. E. Swain, observed that where 

 one of these scrolls came upon a puddle of clear water the 

 crystals were continued in a straight line. Such forms are not 

 at all unusual in the freezing of muddy water, and at the pre- 

 sent moment the puddles in the road opposite my house are 



NO. 1207, VOL. 47] 



the metal will present itself in the form of curved crystals, if 

 the term be allowable. A pretty spray of gold of this character 

 is figured in the report of my lecture " On the crystallization of 

 silver, gold and other metals," in the proceedings of the Royal 

 Institution, vi., 428. If a piece of cuprous oxide be immersed 

 in a solution of nitrate of silver, there shoot from its surface 

 thin threads of silver, which, after proceeding straight forward 

 for a while, suddenly turn at an angle of 120° or 60°, and make 

 perhaps many other deviations : but sometimes these threads, 

 instead of being straight, are curved ; and in that case the 

 threads that branch from them are curved likewise. A magni- 

 fied drawing of such a formation is given herewith. These 

 strange departures from the usual rectilinear course of crystal 

 formation are very curious, and deserve more study than has 

 hitherto been given them. J. H. Gladstone. 



17, Pembridge-square, December 10. 



Prof. Meldola's letter (p. 125) has been interesting to me, 

 as I noted a striking and similar phenomenon here on Thurs- 

 day, December 8, in the forenoon. The trottoirs of several 

 streets (east, west, north and south) were covered all over with 

 beautiful patterns, somewhat difTerent from Prof. Meldola's 

 illustration, there b=ing innumerable dark, broad, sharply-con- 

 toured leaf-like patches, distant several inches from each other, 

 and connected by finely curved and branched tendril-like stalks. 

 Foggy, with a faint north breeze. I should presume the 

 "leaves " were due to spirse drops of sleet fallen during the night, 



Freiburg, Badenia, December 10. D. Wetterhan. 



The graceful arborescent frost patterns described by Prof. 

 Meldola in last week's Nature were very conspicuous on the 

 foot-bridge by the side of Charing Cross railway bridge, on the 

 same morning, this being a situation still more exposed to the 

 wind which he mentions as the probable cause. 



December 12. J. T. Richards. 



I OBSERVED the same phenomenon as Prof. Meldola describes 

 in Nature of December 8, on the same date, December 4, on 

 pavements in Cheltenham, about 10.45 ^-"i- 5 ^f'^r mid-day 

 they had gone. I saw the patterns on pavements running north 

 and south, as well as east and west. They were most exquisite ; 

 some like the illustration, others much more minute ; but always 

 in a connected design over the whole flag. They had all the 

 appearance of fossil vegetation. I never saw anything of the 

 kind before. J. J. Armitage. 



December 13. 



Mr. a. W. Bennett and Mr. E. L. Garbett have also sent 

 communications corroborating the phenomenon observed by 

 Prof. Meldola. The former attributes it to "defoliation of the 

 stones as the result of weathering or wear." 



