REPORT OF THE KEW COMMITTEE. XXX111 



tlie most practicable method of examining sextants, and otlier instruments 

 for geographical purposes. Considering that these instruments, after having 

 been once adjusted, are liable to two distinct classes of error, the one constant 

 for any given reading, and the other variable, it is an object to form Tables 

 of Corrections for the constant errors of instruments sent for examination, 

 and also to ascertain the amount of variable errors which might affect their 

 readings. 



As a groundwork for examination, it is found that small mirrors may be 

 permanently adjusted, at the distance of half a mile, so that when the rays 

 of a mirror of moderate size, standing by the side of an assistant, are flashed 

 upon them, they may re-reflect a brilliant star of solar light, towards the 

 sextant under examination. 



By having four permanently fixed mirrors of (his description, separated by 

 intervals of 20°, 60°, and 40° respectively, and by flashing upon them with 

 two looking-glasses of moderate size, it is possible, by using every combina- 

 tion of these angles, to measure every twentieth degree, from 0° up to 120°. 



The disturbing effects of parallax are eliminated without difficulty, by 

 mere attention to the way in which the sextant is laid on the table, or, in 

 the case of a zero determination, by a simple calculation. 



Moreover, the brilliancy of the permanent mirrors is perfectly under con- 

 trol, by the interposition of gauze shades in front of the looking-glasses that 

 flash upon them. This renders an examination of the coloured shades a 

 matter of great ease and certainty. 



Based upon these principles, Mr, Galton has drawn up a system for the 

 thorough examination of sextants. Each would not occupy more than two 

 hours in having its constant errors tabulated, and its variable errors deter- 

 mined; nor would an outlay of more than £!30 be required for the establish- 

 ment of fixed tables and permanent marks. Difficulty is, however, felt in 

 setting the system in action, owing to the absolute need of an assistant 

 having leisure to undertake it. 



The sum of £179 12*. 6d. has been received from the R,oyal Society, 

 to defray the expense of erecting a model house for the reception of the 

 instruments for Colonial Magnetic Observatories. 



The Photoheliograph has been an occasional source of occupation to the 

 mechanical assistant; but before daily records of the sun's disk can be ob- 

 tained, it is absolutely requisite that an assistant should be appointed to aid 

 Mr. Beckley, because his duties are of such a nature as to prevent his de- 

 voting attention at fixed periods of the day to an object requiring so much 

 preparation as is the case with photoheliogrnphy. Unfortunately, the funds 

 at the disposal of the Committee are quite inadequate for this purpose; and 

 unless a special grant be obtained, the Photoheliograph will remain very little 

 used. 



At present Mr. Beckley is preparing the instrument, under Mr. De la Rue's 

 direction, for its intended trip to Spain, for the purpose of photographing the 

 eclipse which takes place on July 18th. The expenses of these preparations, 

 and of the assistants who will accompany Mr. De la Rue, will be defrayed 

 out of the grant of the Royal Society for that object. 



The requisite preparations are somewhat extensive ; for it has been deemed 

 necessary to construct a wooden observatory, and to make a new iron pillar 

 to support the instrument, adapted to the latitude of the proposed station : 

 both the observatory and iron pillar may be taken to pieces to facilitate their 

 transport. 



The wooden house is 8 feet 6 inches square, and 7 feet high ; it is entirely 

 open at the top, except that portion divided off for a photographic room, 



1860. c 



