Solar Observation, 435 



piece.* It is, of course, intended as an auxiliary to high 

 powers, with which the diminution of light at the same time 

 admits of the advantage of a thin pale screen-glass. In some 

 instances of very large spots and a favourable state of air, the 

 whole of the luminous surface may be excluded, and the spot, 

 under clockwork, or very careful hand motion, be studied alone. 

 This admirable contrivance is also useful without the dark 

 glass for examining many other celestial phenomena where the 

 brightness of neighbouring objects has a disturbing effect. In 

 its latest modification, the various perforations and screens are 

 arranged on circular plates, technically known as "wheels." 

 Lassell's suggestion, that thick paper, covered with white lead, 

 such as is. used for glazed visiting cards, forms an admirable 

 insulator as to heat, may be turned to good account in the con- 

 struction of an economical arrangement to answer the same 

 purpose. 



There is, however, a totally different mode of observation, 

 which, if less striking, and less adapted for minute details, than 

 direct vision, is far more easy and convenient — that of projec- 

 tion ; in which the image is transmitted through an ordinary 

 eye-piece, adjusted by trial till perfect distinctness is obtained, 

 to a large opaque screen at a suitable distance behind it. If this 

 screen is white, smooth, and carefully arranged at right angles 

 to the axis of the telescope, the correct focus being also care- 

 fully determined by repeated trial, this method will give a 

 very fair representation of the principal solar phenomena. Mr. 

 Howlett, indeed, who makes great and successful use of it, tells 

 us that he even gets a more perfect view in this way than by 

 direct vision. At the same time, it has the great merit of sup- 

 plying us with an accurate and inexpensive micrometer, the 

 image of the sun being made, by proper adjustment, to coincide 

 with a circle graduated by lines into suitable divisions ; and 

 thus the position of the spots may be measured, and their pro- 

 gress made evident, from day to day. Carrington, one of our 

 best solar observers, employed this mode, projecting the image 

 on plate-glass, coated with " distemper" of a pale straw colour. 

 A large piece of cardboard, with a hole in the middle, to slip 

 over the object-end of the telescope in the place of the brass 

 cap, must be provided to throw a shade upon the screen ; and 

 the latter, if measurement is the object, must be attached to a 

 bar made fast to the telescope, and partaking of its motion. 



Hornstein and Howlett, by inserting in tho focus of the eye- 

 piece, which for this purpose should be of the " positive" or 



* Mr. Bird, however, with a mirror of only half tho diameter, but reflecting 

 more light in proportion from its silvered surface, found the brasswork greatly 

 heated in about fifteen minutes. Much probably depends on the mode in which 

 the apparatus is constructed. 



VOL. V. — NO. VI. G a 



