length fifty inches. In this camera we exposed four plates. These three 

 cameras, together with a spectroscope, were mounted on a wooden polar 

 axis that was built at the camp. 



The objective of the fourth camera lias a diameter of nine inches and 

 a focal length of 60 feet. This lens was constructed by Mr. O. L. Petit- 

 didier. The front lens is of the ordinary crown glass, and the back lens 

 of a boro-silicate flint. Quoting Fetitdidier from a letter to the writer: 

 "From the point of view of constants they (the pieces of glass) leave 

 nothing to be desired, as the proportional dispersion is practically the 

 same in all parts of the spectrum, so that we should have a perfect 

 achromatic.'' When the samples of the boro-silicate came, however, it 

 was found that it had a decidedly yellow tinge. It was found also that 

 its composition was unstable, and that it oxidized very rapidly in the 

 presence of moisture. After a conference with Mr. Petitdidier, however, 

 we decided to have our lens made of the boro-silicate flint, and to seal it 

 in an air-tight box as soon as it was finished, and to open the box only 

 a few days before the eclipse. Petitdidier had much difficulty in polishing 

 the lens, owing to the fact that it oxidized so rapidly. He found after 

 much experimenting, a solution that would remove the oxidation without 

 affecting the surface. It was with some misgiving that we shipped the 

 lens, but we found on opening it that it had not tarnished in two months, 

 and the surface on the day of the eclipse was as perfect as the day the 

 lens was finished. The air was very humid on the days following the 

 eclipse, and the boro-silicate flint had begun to tarnish slightly when the 

 lens was packed for shipment home. 



This camera was mounted horizontally and fed with a coelostat. A 

 light-tight tube, the outer and inner walls of which were of white canvas 

 and building paper respectively, and which were separated four inches, 

 led from the objective to a dark room in which the plates were exposed. 

 Neither the plates nor the lens was in contact with the tube. The entire 

 instrument was covered with an A tent of white canvas. The plate- 

 holders containing the plates were fastened to a large hexagon, which 

 the operator could revolve at will upon an axis w r hich was parallel to the 

 earth's axis. It was provided with a stop which enabled the operator to 

 bring the plates for the successive exposures quickly and accurately into 

 position. All the slides had been drawn from the plate holders before 

 totality began. The hexagon as well as most of the mechanical parts of 



