522 



NATURE 



\Oct. 29, 1874 



state, they generally have a lively expectation. A squatter, M. 

 De Becker, who lived many years at a remote station, where the 

 blacks were in frequent communication with him, told me he 

 had seen many of them die with a cheerful anticipation of being 

 soon in a "better country." William Ridley 



Paddirgton, Sydney, Australia, July 1 1 



Reported Discovery of Gold in Samoa 



From a note in Nature (vol. ix. p. 273) I am surprised to 

 learn that Mr. Williams, ll.M.'s Consul in these island.s, has 

 stated, in an official despatch to the Foreign Secretary, that 

 gold in quartz has been found on Upolu, in a valley about 

 three miles from the Port of xVpia. The samples assayed are 

 said to have yielded at the rate of 3,000 ozs. to the ton. 



No geologist who knows Samoa will believe tliat gold in 

 paying quantities has been found in this Island. Still, I think it 

 right to give the following explanation of what gave rise to the 

 above report. 



A few months ago gold was said to have been found, as 

 reported by Mr. Williams. Most people here, however, disbe- 

 lieved it, thinking the report had been raised by unprincipled 

 men for the purpose of attracting settlers and promoting the sale 

 of land. Some believed the pretended specimens of Samoan 

 gold had not been found in Samoa, and lelt quite certain they 

 had not been procured in the particular valley specified. 



The facts of the case have been lately disclosed, since Mr. 

 Williams left the islands in ill health ; he was therefore in igno- 

 rance of them when he wrote his despatch from Sydney in 

 October 1873. 



The specimens of gold assayed were brought from the Thames 

 gold diggings in New Zealand, and two or three foreign settlers 

 here, who own land in the valley where the gold is said to have 

 been found, raised the report in order to sell their land at a high 

 price. They appear to have imposed upon the credulity of the 

 Consul, who took the specimens to Sydney and had them 

 assayed there. S. J. Whitmee 



Upolu, Samoa, June 2 



Photographic Irradiation 



1 SHALL be obliged if you will allow me space to state more 

 specifically why I am not able to concur in the irradiation theory 

 of Mr. Aitken (vol. x. p. 439). I understand from his last letter 

 that he fully agrees with Lord Lindsay and myself as to the cause 

 of the outer irradiation, and our only difference of opinion now lies 

 in the amount of the inner irradiation that can be traced as due to 

 what he has termed moleiitlar rcflcctii'ii within the thickness of 

 the collodion film. Mr. Aitken and Capt. Abney both appear to 

 consider this as the chief cause of the inner irradiation fringe, 

 while I am disposed to rank the irradiation arising from the 

 optical impel fections of the instrument with which the photo- 

 graph is taken ; together with any irradiation that may arise in 

 the wet plate processes from circulation in the film of fluid 

 covering the plate — before — or as very much greater in amount 

 than the irradiation due to dispersion within the collodion film. 



We should expect that light dispersed within the thickness of 

 the collodion film would produce its photographic effect in all 

 directions round the illuminated point — and that the area of 

 action would not be affected, or certainly would not be decreased, 

 by covering the front surface of the portions of the collodion film 

 adjacent to the directly illuminated area with an opaque object. 

 Indeed, if the opaque object were a good reflector, such as a 

 bright pjiece of platinum foil, we might expect slightly to increase 

 the area of photographic action due to dispersion within the film ; 

 for the light dispersed towards the front surface of the film 

 would be in great measure reflected back into the thickness of 

 the collodion. But, as I have shown in former letters, placing a 

 piece of platiiumi foil in inmiediate contact with the collodion 

 film causes the photographic image of a bright image to be 

 sharply cut off, and no perceptible irradiation can be traced under 

 the edge of the foil. 



Again, we should expect the action of dispersed light to 

 extend further within a thick film of collodion than within a thin 

 film ; for there would be a greater thickness of illuminated 

 collodion, and the angle through which light could be radiated 

 directly upon the adjacent area without suffering reflection at 

 either surface would be increased, but I have not been able to 

 detect any perceptible difference in the amount of irradiation ol 

 similarly exposed jilates coated with four thicknesses of collodion 

 and in those coated with but one film. 



I have felt myself therefore dri\en to look for the cause of 

 irradiation either in some circulation taking place within the film 

 of liquid covering the collodion at the time of exposure, which 

 film would be interrupted and its tension greatly altered by the 

 contact of a solid body ; or else to seek its exjilanation in the 

 optical imperfections of the photographic instrument Possibly, 

 in the wet-plate processes, circulation within the fluid film may 

 produce a very sensible effect. Indeed, there are phenomena 

 which make this more than probable. When a wet-plate picture 

 of a strong light projected upon a dark background is taken with 

 a decided over-exposure of say ten minutes or a quarter of an 

 hour, the inner irradiation fringe is seen to be most opaque on 

 its outer edge ; and the phenomenon is so marked that it cannot 

 be held to be an elTect of contrast. This, of course, should not be 

 the case if the irradiation edge were due mer^dy to the optical 

 imperfections of the instrument. Again, in the small negatives 

 of the eclipse of December 1S71, taken at Dodabettaand Baikul, 

 there is a decided structure in the irradiation under the promi- 

 nences : under the brightest of them it can be distinctly seen that 

 the opacity of the irradiation fringe is greatest along lines radi- 

 ating from the prominences — while along the outside, that is, 

 furthest from the prominences, there is an arc of slightly greater 

 intensity. The same structure is traceable in all the negatives, 

 but it is most marked in the Baikul series, and especially in those 

 negatives in which the prominences are most exposed, viz., on 

 the east and west limbs, at the beginning and at the end of 

 totality. This, of course, cannot be accounted for merely by the 

 optical imperfection theory. Again, the little brushes mentioned 

 in a former letter as extending under the edge of the platinum 

 foil, cannot be accounted for without supposing that there is cir- 

 culation within the liquid film. I hope 011 my return to England 

 to carry out some further experiments for determining the amount 

 of the inner irradiation which in the wet-plate processes may be 

 due to such circulation. A. Cowper Ranyard 



Florence 



Curious Rainbow 



The unusual phenomena described by Mr. Swettenham as 

 having been observed by him in a rainbow in the Kyles of Bute 

 (Nature, vol. x. p. 39S), are due, I think, to interference. If I 

 remember rightly, he will find an explanation of the matter in 

 " Deschanel's Natural Philosophy," by Prof. Everett. 



Clifton, Bristol, Oct. 19 G. J. Thomson 



Aurora 



A BRIGHT display of aurora was seen here on Friday, Oct. 16, 

 between eight and eleven o'clock. At ten o'clock, when I first 

 saw it, the arch of the aurora stretched from Pollux to Arcturus, 

 then both near the horizon, the apex of the arch being undi r 

 Ursa Major. Deep fringes of light hung from the inner side of 

 the arch and moved with a curtain-like motion to the north. 

 The light was white. On Saturday night numerous streamers 

 were seen darting upwards from the horizon ; and many falling 

 stars, two of them leaving trains of light for about a second. 

 James S. Anderson 



Castletown, Caithness, N.B. 



Sneezing in Animals 



I have a rough-coated terrier dog which will sneeze when told 

 to do so. I taught him this trick by repeatedly imitating sneezing 

 in his presence. 



When about to perform, he shakes his head obliquely once or 

 twice, just as many people do, and then ends with a good sharp 

 sneeze. ' J. F. M. H. S. 



THE RECENT ERUPTION OF ETNA 



pROF. ORAZIO SILVESTRI has published* his 

 •^ observations on the eruption of Etna which oc- 

 curred on the 29th of August, and reminds us that two 

 months previously he predicted not only the formation of 

 the fissure on the Mongibello side, but likewise the 

 eruption by which it was accompanied. 



After an uninterrupted period of eruptive phenomena 

 by which the central crater was considerably modified, at 



* " Notizie sull.i eruzione dell' Etna del 29 Agosto 1874." C.it.-nii.i, 1S74. 



