XXlll 



to enable Mr. Johnson to devote himself almost exclusively to its use 

 without interrupting the pursuits of which we have already spoken, 

 granted him an additional assistant. After a careful study of the 

 peculiarities of the instrument, in the course of which his acquaint- 

 ance with German enabled him to derive most important aid from 

 the learned disquisitions of Bessel, Hansen, Wichman, and other 

 astronomers, an elaborate description of it appeared in the preface to 

 vol. xi. of the Radcliffe Observations. The detection and treatment 

 of certain corrections peculiar to the Oxford heliometer afforded a 

 fine example of Mr. Johnson's suggestive genius. Before com- 

 mencing the more difficult investigations of stellar parallaxes, he 

 passed, step by step, through a patient and judicious course of 

 training, by the measurement of some well-known double stars, of the 

 diameters of the planets, and of the brighter stars in the Pleiades. 

 In 1851 the heliometer was employed in a novel and purely original 

 manner to determine the light-ratios used by different astronomers 

 in their estimations of the magnitudes of the fixed stars. In the 

 two following years his most successful achievements with this instru r 

 ment were accomplished, viz. the determinations of the parallaxes 

 of the stars 61 Cygni and 1830 Groombridge. His near agreement 

 with the values obtained by Bessel for the former, and by Professor 

 Peters and Otto Struve for the latter object, was most satisfactory, 

 and gave ample evidence of his complete success in these intricate 

 investigations. In 1854 and 1855 the parallaxes of Castor, Arc- 

 turus, a Lyree, and one of the comparison stars previously used 

 in the case of 1830 Groombridge, became the objects of researches 

 the details of which are given in vol. xvi. of the Radcliffe Observa- 

 tions. 



The same spirit of enterprise and progress which Mr. Johnson 

 evinced in the purely astronomical part of his duties was manifested 

 in a yet more marked degree in his management of the meteorolo- 

 gical department, From a single page of ordinary records of the 

 barometer and thermometer, the subject rapidly expanded, under his 

 treatment, to an extent hitherto unprecedented. The introduction 

 of photography as a means of barometric registration by Mr. Ronalds, 

 and its ingenious extension to the wet- and dry-bulb thermometers by 

 Mr. Crookes, as also, at a later period, to the records of the direc- 

 tion and strength of the wind and the depth and time of fall of rain, 



