220 ANNUAL OF SCIENTIFIC DISCOVERS 



Bested by Sir John Herschel,* has recently been constructed and placed in 

 the observatory at Kew. The object-glass of this instrument is three and 

 four-tenth inches aperture, and fifty inches focal length; it is not corrected 

 for achromatism in the ordinary manner, but so as to produce' a coincidence 

 of the visual and photogenic foci. The secondary objectives for magnifying 

 the image produced by the principal object-glass are of the Huygheninn 

 form. They are three in number, producing respectively images of the sun, 

 three, four and eight inches in diameter. Between the two lenses of each 

 of these secondary object-glasses, is inserted a diaphragm plate, carrying the 

 fixed micrometer wires, which are of platinum ; these wires are four in num- 

 ber, two at right angles to the other two. One of the wires of each pair is 

 in such a position that they may both be made tangential to the sun's image, 

 while the other two cross at a point situated near the sun's centre. By 

 means of these wires, the distance in arc between each pair having been 

 once for all ascertained astronomically for each secondary object-glass, it will 

 be easy to determine all the data necessary for ascertaining the relative mag- 

 nitudes and positions of the sun's spots. These micrometer wires are under 

 the influence of springs, so as to preserve a tension upon them when ex- 

 panded by the sun's heat, and thus to keep them straight. The principal 

 and secondary object-glasses are not mounted in an ordinary cylindrical 

 tube, but in a pyramidal trunk square in section, five inches in the side at 

 the upper end, Avhich carries the principal object-glass, and twelve inches in 

 the side at the lower end, which carries the photographic plate-holder and 

 the usual ground glass screen for focusing. This trunk is firmly supported 

 by a declination axis of hard gun metal two and a half inches in diameter; 

 it is furnished with a declination circle ten inches in diameter, reading to one 

 minute of arc, and has a clamp and screw motion for fine adjustment in 

 declination. 



The polar axis is driven by a clock driver, which answers perfectly, and is 

 easy of regulation to the greatest nicety, so that the sun's limb remains for 

 a long period in contact with the tangential wires. 



The polar axis of the telescope is carried by a dial-plate, which fits on the 

 top of a hollow column of cast iron, the section of which is a parallelogram. 

 This column is securely fastened to the stone foundation. The instrument 

 is mounted within the rotating dome of the Kew Observatory. 



The telescope and its mechanical appliances may be said to be perfect so 

 far as they go, but experience will undoubtedly suggest several minor alter- 

 ations and additions before the telescope is brought practically to work. 

 The photographing of such minute objects as the sun's spots will require 

 at all times the utmost skill and care of an accomplished photographer, even 

 when the telescope has been fairly started. The difficulties yet to be mas- 

 tered must occupy some considerable time. The first attempts have been 

 confined to the production of negative photographs, but in consequence of 

 the imperfections always existing in the collodion film, it has been deemed 

 advisable to make attempts to produce positive pictures, and recourse may 

 ultimately have to be made to the daguerreotype process. 



* See Annual of Scientific Discovery, 1855, p. 203. 



