June, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



119 



of greater length than the first; these are the second 

 order spectra. 



It may happen that the two spectra of the first order 

 cannot both be made to fall on the ground glass, be- 

 cause the plate is not sufficiently large. In this case 

 no attempt should he made to photograph both spectra, 

 but the solar image should be moved a little to one 

 side of the centre, and one of the spectra made to 

 appear on the plate. 



To take fuller advantage of the grating, especially 

 on such an occasion as an eclipse of the sun, more en- 

 terprising observers can easily construct a special form 

 of box camera. By this means both the corona and 

 the first and second order spectra can be secured. 



Such a camera as this is shown in the accompany- 

 ing illustrations (Figs. 4 and 5), and was used with 

 successful results in the Spanish eclipse of 1900. Even 

 the form here given did not take the fullest advantage 

 ol the grating, because it was arranged only to photo- 

 graph the direct image of the corona, and the two 

 orders on one side of this image. 



In the figures it will be noticed that the camera is 

 constructed flat, the lines radiating from the lens 

 (shown on the top of the box) indicating the directions 

 of the incident light falling on the photographic plate. 

 The thicker line on the right (Fig. 5) represents a 

 diaphragm to cut off the reflections from the side of 

 the box of the first and second order spectra that were 



on the photographic plate and so spoil the result. For 



such exposures, therefore, very fast plates should be 



Fig, s- The same Camera 

 above. The white lines in 

 the Corona and the Spectra. 



as illustrated in Fig. 4, but seen from 

 dicate the directions of the images of 



used, and the spectrum plate is one to be recommended. 

 It is hoped that by employing gratings in this man- 



Fig. 6.— The Corona and first order Spectrum as photographed with the Camera shown in Figs. 4 and 5. Enlarged 



twice. Exposure 6s seconds. 



not required. The focussing of the camera was ac- 

 complished by making the lens capable of being moved 

 in and out of the box. It may be remarked that the 

 grating in front of the lens makes no difference to the 

 ordinary focal length of the lens, so that cameras which 

 are tocussed according to distance require simply to 

 be set for infinity, as if an ordinary landscape was 

 being- taken. 



Fig. 6 will, perhaps, give the reader some idea of 

 the nature of the picture he will secure, but in this case 

 only the first order spectrum has been reproduced. 

 Owing to the fact that the solar light was thrown on 

 to the grating by means of a siderostat, it was possible 

 to give a long exposure to the eclipsed sun; for this 

 example the exposure lasted 65 seconds. 



Unfortunately, the plate was begun to be exposed 

 before the chromosphere was covered by the moon, so 

 that the spectrum in this instance is chiefly chromo- 

 spheric. Nevertheless, the green ring forms a very 

 conspicuous feature in the original negative. 



In cases in which the camera is not equatorially 

 mounted or used in connection with a siderostat, helio- 

 stat or coelostat, the exposures have to be restricted to 

 about 20 seconds, otherwise the apparent movement 

 of the sun would cause the image to change its position 



ner a new interest will be given to the user of small 

 cameras, and that they will be rewarded with results 

 that may prove of service to the cause of Solar Physics. 



The Action of Wood on 



Photographic Plates 



in the DdLrk. 



By C. Ai.N'swoRTH Mitchell, B.A. (O.xon.), F.I.C. 



Some years ago Mr. T. C. Hepworth informed the 

 writer that he had taken away some plates wrapped in 

 dark paper in a wooden box, and that to his surprise 

 many of them were " fogged " when unpacked a few 

 days later. The phenomenon appeared inexplicable 

 until in 1897 Dr. W. J. Russell showed that turpentine 

 vapour had a pronounced darkening action upon a 

 photographic plate, and in 1899 contributed a paper to 

 the Royal Society in w^hich it was shown that many 

 other hydro-carbons, such as resins, had th'j same 



