KEPORT OF THE SECRETARY. 108 



A shade glass opaque to violet light was placed over this lens. The purpose of 

 the shade glass was to enable a comparison to be made between the form of the outer 

 corona as photographed with yellow and green light and as photographed with the 

 complete coronal radiations by other lenses shortly to be described. 



Within the eastern part of the shed there was mounted upon an improvised polar 

 axis a collection of four cameras, quite ponderous in appearance, as indicated in 

 Plate XI, but really not very heavy, and well provided as to moving gear by being 

 connected with the very accurate spectrobolometer clock. These cameras were two 

 similar pairs, one with short-focus, the other with long-focus lenses. The former 

 were tw(j landscape lenses of 4h inches aj^erture and 40 inches focus, each provided 

 with a 30-inch square plate. In front of one lens was placed a shade glass opaque to 

 violet light. The two long-focus lenses were of 3 inches aperture and 11 feet focus, 

 and were thus like those recommended in the Harvard College Observatory Circular 

 No. 48 as most suitable for a photographic search for a possible intramercurial planet. 

 The axes of these two cameras were inclined so that together they covered a space 

 east and west of the sun about 12° by 28° in extent. Their fields were found to be 

 so nearly flat as to make it undesirable to use a nest of plates arranged upon a curved 

 surface, as recommended in the Harvard circular above alluded to, and each camera 

 had a single plate 24 by 30 inches. 



All the photographs with the seven lenses above described were taken upon Cramer 

 double coated isochromatic plates of great rapidity. 



An automatic camera, giving exposures from a break-circuit chronometer beating 

 seconds, was provided for the purjiose of securing the times of contacts. This camera 

 had a 22-inch lens with pin-hole aperture, and the exposures were made upon very 

 slow nonhalation celluloid plates 15 inches in diameter, rotated slightly after each 

 exposure by an electrical escapement. One plate was provided for first contact, one 

 for both second and third, and one for fourth contact. As no clockwork was applied 

 to move this camera, the successive exposures made a spiral series of images of the 

 sun, from the appearance of which the gradual encroaching of the moon could be 

 observed. 



(b) Apparatus for bolometric purposes: This consisted of a complete holographic 

 outfit, including not only the great Grubb siderostat with supplementary mirror, 

 but also a double-Avalled chamber of nearly uniform temperature. The view of it is 

 shown in Plate XII, and its purpose was to enable the total radiation of the inner corona 

 to be observed, and in addition, if practicable, to determine the distribution of these 

 radiations in the spectrum. The latter observation it was hoped would throw light 

 on the composition of the corona, for it is well known that different substances and 

 different temperatures have each its characteristic energy spectrum. A beam of 

 light from the 17-inch mirror of the great siderostat, reflected due south into the shed, 

 passed through a cat's-eye diaphragm whose aperture was controlled by the observer 

 at the galvanometer, thence to a condensing mirror, which reflected the rays directly 

 back to the focus at 1 meter distance, where was a slit 1 centimeter high and 1 milli- 

 meter wide. From the slit the rays were reflected out of the optic axis of the con- 

 densing mirror by two parallel plane mirrors, and fell upon a coUimating mirror 

 of 75 centimeters focus. Thence they were reflected upon a prism 8 inches in dia- 

 meter, one of whose surfaces was silvered, so that the prism might be used either 

 to refract or reflect the rays, according as the glass or silvered face was turned 

 toward the (,-ollimator. From the prism the rays passed to an image-forming mir- 

 ror, in the focus of which, at 75 centimeters distance, was the bolometer strij^ 1 centi- 

 meter high and 1 millimeter wide. The bolometer and galvanometer with their 

 accessories were essentially as used for solar spectrum work in Washington, and 

 while the optical train, with its seven reflections and small slit, greatly reduced the 

 radiations, the sensitiveness of the bolometer was yet such that subsequent observa- 

 tions on the full moon gave a deflection of 85 divisions when the aperture of the dia- 

 phragm was but 17 centimeters square. 



