254 



NATURE 



\ynly 25, 1872 



of the liglit emerges into the fihn of water, and is then ab- 

 sorbed by the black paper. 



Fig. C rejireseiits a photograph taken upon a yellow glass 

 plate, with a backing of wet black paper, but otherwise exposed 

 under similar conditions to the photograph represented in Fig. Y>. 



•''■The outer irradiation halo may therefore be entirely avoided 

 for the future in any corona or other necessarily over-exposed 

 photographs by the use of the opaque plates. If, however, it 

 is considered important that the negatives should be capable of 

 being copied by transmitted light, the outer halo may still be to 

 a great extent avoided by theuse of transparent glass plates with a 

 backing of wet black paper or black varnish. 



Secondly, as to the inner and more definitely-marked irradia- 

 tion edge which remained and seemed to be unaffected by the 

 precautions that had served to rid us of the outer halo. Since 

 the inner fringe was equally to be found on an opaque and on a 

 transparent plate, we felt ourselves justified in seeking for lis 

 cause in front of the first or upper surface of the prepared pkle ; 

 that is, it must be referred either to some action taking place 

 within tlie thickness of the collodion, or to the optical imperfec- 

 tions of the instrument. 



In order to deteraiine whether the scene of action lay within 

 the thickness of the collodion, we placed an ivory ruler with a 

 bevelled edge in immediate contact with the collodion film. The 

 plate with the ruler upon it was then exposed within the camera, 

 so that the image of an incandescent platinum wire fell partly 

 upon the collodion film, and partly upon the ivory ruler. II the 

 scene of action lay within the collodion film, we might expect 

 the inner irradiation fringe to oxtend itself under the edge of the 

 ruler, while if it were due to the optical imperfections of the lens, 

 the image of the wire would be cut off sharply by the edge of the 

 ruler. 



On removing the plate from the camera, and before the ruler 

 was shifted from its place on the collodion, the whole was ex- 



posed for a few seconds to the action of the light from a gas- 

 burner, in order that the position occupied by the edge of the 

 ruler might be faintly printed upon the collodion film. On de- 

 veloping the plate, it was found that the image of the wire was 

 sharply cutoff at the place occupied by the edge of the ruler, as 

 in Fig. D. 



The very faint action extending inwards under the ruler lieing 

 evidently due to the want of perfect opacity in the ivory, it seems, 

 therefore, to be clearly proved that the inner irradiation edge is 



not caused by any chemical or other action taking place within 

 the thickness of the collodion ; but must be referred to the op^tical 

 imperfections of the instrument which throws the image upon the 

 collodion film. 



It is instructive to remark that the photographic image of the 

 wire is not cut off by an absolutely straight line at the edge of the 

 ruler, but it is slightly convex, and is separated from the faint 

 action which has apparently taken place through the ruler by a 

 very narrow bright line, which appears to indicate the presence 

 of a small capillary film of liquid along the edges of the ruler, 

 forming a minute cylindrical lens. At the point where the collo- 

 dion was acted upon by the light, the minute cylindrical lens ap- 

 pears to have been interfered with, and depressed inwards 

 towards the ruler ; we may therefore conclude that the collodion 

 film is slightly swelled or thickened by the action of the light 

 upon it.* 



The cause of the inner irradiation-edge seems to be that every 

 point of a luminous object is not represented by a simple point 

 of light in the luminous image ; in other words, the circle of 

 least diffusion of any pencil is a curve of sensible area, of which 

 tlie central and most intense portions imprint themselves first 

 upon the collodion. 



Some further experiments were made in order to test whether 

 the size of the circles of least diffusion was chiefly owing to chro- 

 matic aberration (in which case the difficulty might be got rid 

 of by the use of reflectors) jifor this purpose a bath of solution 

 of sulphate of copper was placed in front of a gas-burner, and 

 the triangular diaphragm sho\\n in Fig. A was then placed be- 

 tween tlie sulphate of copper bath and the lens of the camera ; 

 but the blue screen thus formed seemed to have very little effect 

 in altering the breadth of the irradiation-fringe, only slightly re- 

 tarding the rate of its formation ; a similar result was obtained 

 on placing a piece of yellow glass in front of the diaphragm ; 

 in this case, however, the formation of the fringe was still further 

 retarded. 



Photographers have long known that by making use of stops 

 they can obtain a much sharper image. By way of experiment, 

 we cut off the edges of the lens with a circular stop, and found 

 that the inner irradiation-fringe was thus greatly decreased. It 

 seem«, therefore, fair to argue that the aberration of oblique 

 pencils exceeds in magnitude the other disturbing causes, and 

 that it will be well, in making preparations for the photographic 

 observation of the transit of Venus, to avoid as much as possible 

 all oblique pencils. 



We would, therefore, place our photographic plates in the 

 primary focus, and thus avoid the necessarily deep curves of 

 any arrangement of lenses which may be used for enlarging the 

 image. Whether it would be best to make use of a reflector or 

 a refractor, remains to be settled by further experiment, but 

 our present experiences would lead us to vote in favour of the 

 reflector. 



We cannot conclude without returning our best thanks to 

 Mr. H. Davis, who has rendered us able assistance in carrying 

 out the foregoing e.xperiments. 



SCIENTIFIC SERIALS 



Anuiilcn dcr Chcmic und PharinactL', February and March 

 1872. — This double number is unusually bulky ; it contains no 

 less than 252 pages, and abounds with much interesting matter. 

 The effect of the new management seems to be evident, as the 

 last paper published in this number was only received on January 

 14 ; formerly some two or three months gcucr.illy elapsed fruiu 

 the time of the reception of a paper before its publication. The 

 number of the papers renders it impossible in the short space at 

 our disposal to give more than a passing glance at some of the 

 more important. Amongst them we notice three papers in con- 

 tinuation of Linnemann's researches ; these treat of the synthesis 

 of normal butyric acid, of butyl alcohol, and on some of the 



* We are at present unable to find any explanation of the slight apparent 

 thickening of the end of the image of the wire where it abuts upon the ruler, 

 but the same thickening is to be found in all the plates. It may be well to 

 remark that it .appears evident from sHght indication in the negatives which 

 it would be difficult to render in a woodcut, that tlie true edge of the ruler 

 coincides with tlie inner side of the white fine (or with the side away from the 

 image of the wire). The convexity of the end of the ima;>e of the wire can- 

 not, therefore, be regarded as indicating even a slight chemical encroach- 

 ment. The slightly tapered appearance of the other end of the image of the 

 wire is due to the fact that the platinum incandescent wire is cooled at its 

 jioiutsof contact with the thick copper wires of the circuit. 



