KDomledge & SeieDtjf je Nems 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL and E. S. GREW, M.A. 



Vol. II. No. 6. 



[new series.] 



JUNE, 1905. 



r Entered at 

 LStationers' Hall. 



SIXPENCE. 



CONTENTS.— See Page VII. 



Thorp GroLtings a^nd 



SmsLll CacmeraLS in 



Kclipse Work. 



By WiLLi.AM J. S. LocKVER, M.A., Ph.D. 



The total solar eclip.se of Augfust next will, no doubt, 

 attract a large number of people away from these 

 shores, and possibly the great majority will go armed 

 with a kodak or some other form of small camera, in 

 order to bring back a record of the corona. 



It is unfortunately generally considered that the work 

 accomplished with small cameras, that is, those having 

 an aperture of about one inch, has no scientific value, 

 because the employment of larger instruments by the 

 numerous official parties gives all the information that 

 is desired, and in a more eflicient manner. 



During the last few eclipses the small camera has 

 demonstrated that it is capable of obtaining results 

 which cannot be secured with instruments of large 

 dimensions. Perhaps the first most notable instance 

 of this is the success that was achieved in the photo- 

 graphy of the coronal streamers. Thus, in the Indian 

 eclipse of 1898, with lenses of li inches in diameter, 

 and 9 inches focal length, Mrs. Maunder obtained a 

 photograph of the corona, showing one streamer ex- 

 tending to a distance of 12.9 lunar radii from the 

 moon's limb. At the same eclipse, a blue-jacket made 

 exposures with the writer's camera (lens aperture 0.8 

 inches, focal length 8.8 inches) fixed on a stand, and, 

 with an exposure of 15 seconds, obtained a picture 

 showing one streamer extending to 10 lunar radii from 

 the moon's limb. 



A reproduction of this photograph is here shown 

 (Fig. i), but unfortunately the extensions are too 

 delicate to be satisfactorily indicated. 



The special object of the present article is, however, 

 to draw the attention of those who will employ small 

 cameras to the use of Thorp's replicas of Rowland 

 gratings in connection with them. Not only can a pic- 

 ture of the corona be obtained, but ai the same time, and 

 on Ihe same plate, the spectrum of the corona is also 

 secured. This spectrum is in the form of rings, like 

 that obtained with the prismatic cameras. 



It may be said that the spectrum of the corona is 

 obtained best with apparatus of large dimensions. This 

 is true if the spectrum of the chromosphere be inferred, 

 but in the case of the coronal rings, which are faint 

 (with perhaps the exception of the green ring), there 

 seems a great chance of small cameras rendering valu- 

 able assistance. 



The main objects then of using these small instru- 

 ments fitted with these gratings are to give us (a) long 

 coronal streamers (if there be any), and ifi) a record 

 of the coronal rings. With this aim it is therefore 

 best to be well within the shadow-, as near to the central 



he bclipse of 1 89S as photographed with a 

 iches aperture and 8 '8 inches focal length, 

 seen in the right-hand bottom corner. 



line as possible, and to commence the exposures some 

 seconds after totality has begun, and finish the last 

 exposure some five or ten seconds before the end ot 

 totality. This course is suggested to eliminate as fat 

 as possible the chances of photographing the chromo- 

 spheric spectum, which might mask the coronal rings. 



At the present time Thorp gratings can be procured 

 in two sizes, the ruled surfaces covering an area of 

 ij;; by \\ and JJ by J;- inches respectively. The 



former are mounted on selected or worked glass 2\ by 

 2 by is inches in size, while the latter are placed on 

 glass plates 2 by ij by l-, inches. The prices vary 

 from fifteen shillings each to ten and even lower. 



The grating should be fixed square on to the front 

 of the camera lens with the lines of the grating in a 



