40 TRANSIT OF VENUS, 1874. HONOLULU. 



required, with the Transit Clock in the same manner as the Altazimuth 

 Clock. 



Several days before the Transit of Venus, Lieutenants OLDHAM, SHAKESPEAR, 

 and CLAPP, of H.M.S. Scout, kindly attended at the observatories to rehearse 

 the intended operations. Lieut. Oldham was to assist Mr. Nichol, Lieut. 

 Shakespear to assist Mr. Noble, while Lieut. Clapp was to aid me ; and all 

 this assistance was most effective, and enabled the work to be well and 

 quickly done when every minute was precious. 



Every observer had an adjustable seat, so that he could be perfectly at 

 ease. 



At noon Mr. Nichol and I made a complete comparison of all the clocks 

 and chronometers intended for use, and provided for the contingency of any 

 of them stopping at a critical time. 



At l h I carefully adjusted the direct-vision spectroscope on the 4|-inch 

 equatorial. Although I attach very little value to the observation made 

 with the spectroscope, it was sufficiently interesting to be worth recording. 

 The spectroscope was made by Mr. Browning, and consisted of a direct-vision 

 prism, of five components, two flint and three crown cemented togetheiv 

 sufficiently large to transmit a pencil 1'2 inches by 07 inches, with collimator 

 and examining telescope each of 7 inches focal length. The slit was opened 

 and closed by a small micrometer-screw acting against a spring. I deter- 

 mined the value of a revolution of the screw by opening and closing the slit 

 through measured ranges under the microscope. During my observation the 

 slit was open - 0016 of an inch. The power of the combination was about 55 or 

 60 diameters. In the focus of the eye-lens of the small examining telescope 

 I inserted an opaque screen with a narrow slit in it, the width of which was 

 about ^th of an inch, and the length rather more than the breadth of the 

 spectrum. The Fraunhofer line C occupied the middle of this aperture, its 

 apparent width or image of the opening of the slit being about one half of 

 its breadth, so that the entire spectrum was cut off from view except the 

 C line and a narrow strip on either side. The spectroscope was firmly 

 attached to the telescope by a brass tube supporting it at its center of 

 gravity. There was an opening in this tube giving access to the slit, which 

 was of course placed exactly in the primary focus of the object-glass. 



At 2 h . 30 m local mean time I placed the slit tangentially on the Sun's limb 

 at the expected point of first contact. The hydrogen stratum or " chromo- 

 sphere " was well defined. There was no prominence near the point of 

 contact, but the outer limit of the chromosphere was irregular. 



