356 Prof. H. H. Turner ami Mr. II. K. X.-\vall. 



of the 20 seconds' exposure through the thinnest of the square 

 screens. 



23. Development. The plates in slide No. 1, and the corona picture 

 of slide No. 2 (1 second), were developed with amidol : also the plate 

 exposed to the integral photometer mentioned in 12. All the others 

 with metol. 



PART III. SEPARATE RETORT BY H. F. NK \v.\u.. 



24. Tlw Four-prism Spectroscope with Slit. 

 It was intended to attempt 



(i) To secure photographs of the bright-line spectrum of the sun's 

 limb at the beginning and end of totality, five photographs at 

 the beginning, six at the end. 



(ii) To photograph the spectrum of the corona in two separate 

 regions of the corona. 



It had l)een decided not to attempt to determine the velocity of 

 rotation of the corona, for the duration of totality was not long enough 

 to give satisfactory images of the lines in the spectrum of the corona 

 at such distances from the limb as would ensure some measure of 

 certainty that the observations would not deal with the local disturl>- 

 ances known to exist near the chromosphere. 



The instrument arranged for the purposes above mentioned is a four- 

 prism spectroscope with a single slit. It was used by the writer in 

 India, at Pulgaon in 1898.* The only changes made in it were that 

 (i) only one slit was used instead of two ; and (ii) one of the prisms 

 which had been found to give imperfect definition on account of want 

 of homogeneity in the glass had been replaced by another prism. The 

 prism box and train of prisms had been used at the Cambridge 

 Observatory for a star spectrograph, and were dismounted, for use in the 

 eclipse, after the completion of certain observations of Capella. 



The train of prisms is of such dimensions and construction as to 

 transmit a 2-inch beam of light, and to produce a minimum deviation 

 of 180 for H y . The collimator and camera are set parallel to one 

 another. 



The whole spectroscope is mounted so as to turn about an axis 

 parallel to the collimator. The axis is rotated (with a period of 

 twenty-four hours) by clockwork, and is tilted so as to be parallel to 

 the earth's axis. In this position the collimator points to the north 

 pole, and the camera to the south pole. 



The tube of the collimator is prolonged beyond the plane of the slit, 

 and is arranged to carry at its end a mirror of speculum metal and an 



* ' Roy. Soc. Proc.,' rol. 64, p. 55. 



