458 



NATURE 



\_August 29, 1878 



obtained atjiearly all the stations. In order to give an 

 idea of thcextensive preparations which have been made, 

 I may state what were Dr. Draper's photographic appli- 

 ances as an instance of the equipment at one station. 



In the first place there was a telescope of 5 inches 

 aperture, and 78 inches focal length, especially corrected 

 for photography, to get as complete a photograph of the 

 corona as could be obtained by an exposure lasting 



lines which lay in the actinic region of the spectrum, 

 ring-formed images, one ring for each bright line, would 

 be introduced. Cn the other hand, if the light of the 

 corona arose from solid or liquid bodies, or was reflected 

 light from the sun, a long band answering to the actinic 

 region of the spectrum alone would be produced. If the 

 light was partly from gas and partly reflected sunlight, a 

 result partly of rings and partly a band would have been 

 obtained. 



As there was an impression that it was impossible to 

 obtain a photograph of the spectrum of the corona. Dr. 

 Draper, in order to give the best possible chance of 



Fig. 3. — I, image of sun ; 2, first order spectrum ; 3, second order. 



getting such a photograph, resorted to exceedingly sen- 

 sitive materials, known as the lightning collodion process, 

 furnished by the Messrs. Anthony, of New York. This 

 involved a necessity of distilling a large part of the 

 water used, because at Rawlins the water contains either 

 alkali or sulphur, both of which are deleterious. 



Since so many attempts were made, I had better con- 

 fine my attention to the photographs. I have seen those 

 of Prof. Hall and Dr. Draper. I will first deal with 

 those of the corona itself. • The former, taken by a dry- 

 plate process, and, I believe, by means of a large aper- 

 tured camera of short focus, are very admirable, and 



during the whole of totality. This was during 165 

 seconds. 



Next there was a large instrument which may be called 

 a slitless " phototelespectroscope." This instrument con- 

 sisted of an object-glass, composed of four lenses 6 

 inches in diameter and 21 inches focal length. The 

 image of the sun at the focus was less than \ of an 

 inch in diameter, and of extreme brilliancy. Before the 

 rays of this lens reached the focus, however, they were 

 intercepted by a Rutherfurd grating, about 2 inches square, 



2. — Telescope. 



set at an angle of 60 degrees. This threw the rays to one 

 side, and produced three images — a central one of the 

 sun, and on either side of it a spectrum. One of these 

 spectra was dispersed twice as much as the other, that is, 

 would give a photograph of twice the length. This last 

 photograph was actually about 2 inches long. With this 

 instrument, mounted equatorially and driven by clock- 

 work, if the light of the corona was due to gas, giving 



O 



00000 



0000 O 



Fig. 4. 



show the structure at the north and south points most 

 exquisitely. 



One of Dr. Draper's was exposed during the whole of 

 totality, and represents a corona somewhat like those 

 seen in the photographs of the 1871 eclipse taken with a 

 medium exposure. In another, by a fortunate shake of 

 the instrument, a trace of the outline of the chromosphere 

 was received. 



Next I come to the Spectra ; but first I may be per- 

 mitted to refer to the only photographic contrivance 

 which was possible for my light marching order. This was 

 a small portrait camera with a lens of one inch aperture. 



Fig. s- 



and a Rutherfurd grating of about 6,000 lines to the 

 inch in front of it to act as a reflector and disperser. 



I placed the camera and the attached grating on the 

 ground in such a position that on the focusing screen I 

 got the direct image of the sun at one edge of the plate, 

 and running across the plate the spectra of the first and 

 second orders on one side. A rough notion of what is 

 seen on the plate when the sun is photographed thus will 

 be gathered from Fig. 3. 



Now if the corona had been built up of gases compe- 

 tent to give us lines with a spectroscope of ordinary 

 construction, what I ought to get during an eclipse from 



