Jan. 8, 1874J 



NA TURE 



183 



of a lightning-conductor to the ground, was connected with this 

 wire. The charges which this wire indicated as occurring in 

 the straum of atmnsphere witli which it was in contact, were of 

 a most extraordinary character. 



Simultaneously with the occurrence of a flash of lightning, 

 even at a mile riistanct^ (the batteiy being disonnecte'i), a torrent 

 of sparks rushed between the exploding balls, pres-tnting ihe 

 appearance of a thick bunlle of brilliant spirks, with a n ise 

 similar to that of suddenly breaking a hard fibrous stick. When 

 the battery was in connection with the atmosp 'eric wire, the 

 quantity of electricity brought in by it was sufficient to charge 

 and discharge the battery, over an interval of a quarter of an 

 inch several times so rapidly, that it was impossible to count the 

 discharges, the cracking noise being correspondingly loud. This 

 effect is now accurately imitated by the bundle of sparks 

 passing between the terminals of a powerful induction 

 coil, having an electrical battery connected with it. When 

 it is considered that the earth's surface in immediate con- 

 nection with this electrified stratum is acted upon electrically by 

 it, one can easily see the influences which such actions are likely 

 to produce upon the delicate vegetable organisms uhich have 

 not only to act as conduotors in these electrical changes, but are 

 exposed to the actions of a sudden development of au enormous 

 amount of ozone. It will therefore be an interesting matter to 

 know how vegetable life will be influenced by an ozonised at- 

 mosphere, espt^cially as the conditions necessary for artificial ex- 

 periments will not be difficult to obtain. 



Plymouth, Dec. 29, 1873 J. N. Hearder 



Photographing the Transit of Venus 



The following is the result of some experiments recently made 

 on photographic irradiation : — 



If, as is generally supposed, photographic irradiation is caused 

 by the reflection ol light from the back surface of the plate, then 

 photographs taken on non-actinic coloured glass ought to be free 

 from irradiation, because the light would be quenched in the 

 glass, and therefore no reflection could take place. Photographs 

 of a model transit were taken on yellow, orange, and red 

 glasses ; but in all cases the irradiation was nearly as bad on 

 the coloured glasses as on the clear glass. 



Photographic irradiation may possibly be caused, either by 

 the bright light producing an intense state of chemical activity, 

 which has the power of spreading itself, or what seems more 

 probable, the parts of the collodion on which the bright light is 

 falling become luminous, and reflect light to the surrounding 

 parts of the sensitive film, and thus extend ttie chemical change 

 in each side of the true optical boundary litie. If this is the 

 explanation, then we can correct photograp'iic irradiation by 

 allowing only sufficient light to fall on the plate to produce the 

 necessary chemical change, so that there shall be no surplus to 

 be reflected ; o,' we may make the sensitive film of such a nature 

 that it cannot reflect the actinic tay. There are two ways of 

 carrying out the first of these plans. We may either "stop" 

 down the lens by means of a diaphragm, or we may pa-s the 

 light through a non-actinic col >ured screen. The first should be 

 the best plan, but was not found practicable with the Dallmeyer 

 "triplet" lens used in the experiments. Scr.-ens of glass and 

 coloured solutions were then tried, and photographs of the model 

 transit taken perfectly free from irradiation, and not to be dis- 

 tinguished from photographs of the model taken against a dull 

 sky, which required 15 seconds' exposure. Experiments were 

 then made to make the sensitive film incapable of reflecting ac- 

 tinic rays. This was done by adding red aniline to the collo- 

 dion, till the colour was found by experiment to be deep enough. 

 Photographs taken in this way were also quite free from irradia- 

 tion. After the photographs were developed and fixed in the 

 usual way, they were treated with chlorine gas, which destroyed 

 the red colour and left the photographs on a clear film. 



Ocular irradiation is also, in all probability, in part caused by 

 the reflection ot light in the eye. But m ad.lition to this cause 

 there is another of considerable importance — namely the " per- 

 sistence of the image '' combined with the unconscious motion of 

 the eye — as the impression received by the brain is not only that 

 of the light on the part of the retina where the image at the 

 time is, but also that of where it was a short time before, 

 the mental impression must therefore be larger than the 

 image on the retina. Ocular irradiation can also in all proba- 

 bility be corrected, by reducimi the amount of light falling on 

 the eye, to the minimuoa necessary to give a distinct impression. 

 The reflection in the eye will then be less. The image not 



being so bright will not " persist " so long — and the light not 

 being so brilliant, the stimulus to the unconscious motion of the 

 eye will not be so great. Diaphragms will of course be pre- 

 ferred for ttiis purpose. When screens are used it is probable 

 that neutral tinted ones will be found to suit best. 



John Aitken 



The New Marine Animal 



In Nature, vol. viii. p. 4S8, under the heading "New 

 Marine Animal from Washington Territory," Mr. P. L. Sclater 

 announces the description by Mr. .Stearns of the VerriUia blakei, 

 the long-sought-for owner of the wand-like rod named by Gray, 

 OsUocella septentrioiiah'. 



I write to say that the nationality of the Polyp is altogether 

 British ; Burrard's Inlet — the only place it has yet been found — 

 is in British Columbia, close to the north mouth of the Fraser, 

 and the first description of it would have been British too, but 

 for unavoidable postal delays in the transmission of my paper, 

 the receipt of which by the Zoological Society Mr. Sclater men- 

 tions. Edward L. Moss 



Royal Naval Hospital, Esquimalt, B.C., Nov. 26, 1873 



The Potato Disease 



In N.^ture, vol. ix. p. 161, it is stated by Mr. W. G. Smith 

 that the bodies referred by Ur. Montague to Artotrogus are pos- 

 sibly no other than ]'o!iitiUa ciliata. Nothing can be more com- 

 mon on decaying potatoes than V. ciliata, but I can state most posi- 

 tively that Montague's fungus, whatever its nature may really be, 

 had nothing to do with V. ciliata. It is very important that at- 

 tention should not be drawn off from Dr. Montague's, or rather 

 Dr. Rayer's curious observatirjn by a supposition which is en- 

 thely without foundation. A reference to the figures in the 

 Journal of the Horticultural Society (vol. i. tab. 4, figs. 27, 28, 

 29), and the chaiacters of Artotrogus, apart from the specimens 

 submitted to myself, and the occurrence within the cellular tissue, 

 ought to be quite sufficient. 



Jan. 3 M. J. Berkeley 



Specific Gravity of Sea-water 



In Prof. Wyville Thomson's work "The Depths of the Sea" 

 there appears to me a curious discrepancy between two state- 

 ments of the specific gravity of t.le sea, to which it may be 

 useful to direct general attention. At p. 505, Mr. W. L. 

 Carpenter states that the average specific gravity of surface- 

 water, at a sufficient distance from land to be unaffected by local 

 disturbances, was I '02779. At p. 513, Dr. Frankland gi/es the 

 specific gravity of four samples ol surface-water, the mean of 

 which is only I 'O267, even less than the minimum value as 

 given by Mr. Carpenter. Both resuhs are said to be for tempera- 

 ture 60° F. I should have expected Dr. Frankland's determina- 

 tion to have been ths higher, from possible loss by evaporation. 

 The difference may pro lably be due to want of identity of indi- 

 cation between the instruments used. From whatever cause it 

 may arise, the difference is so considerable, as to leave no doubt 

 whatever that it ought to be accounted for in some way ; and the 

 error wherever it lies fully exposed. R. Strachan 



Meteorolog cal Office 



Optical Phenomenon 

 A short time ago I was lying, during the heat of the day, in 

 a darkened room in a house at one of the hottest stations in 

 India. There was a great glare of sunlight outside. All at 

 once I became aware of figures moving about on the opposite 

 waU. On examination they proved to be the inverted images of 

 the servants of the establi-hment who were walking about in 

 the per ormance of their several duties in the gravelled court- 

 yard outside the hoase. The white colour of their cbthes, the 

 dark colour of their slcin, and the ted colour ol their sashes or 

 turbans, were distinctly reproduced, and every servant was 

 recognisable without difliculty. The images were produced by 

 rays passing througa tnree or four holes in the Venetian shut- 

 ters ; and while they all remained open there w.as a large penum- 

 bra round the images, but on closing all but one hole, this was 

 very much reduce:!. The holes were of the size of a shilling or 

 half-crown, and made in an outer door as well as the shutter, 

 having been constructed to admit of a punkah rope passing 

 through. The explanation appears to be this :— The sun was 



