Solar Observation. 297 



and purple and green nave been used by others.* From the 

 great convenience of being able to vary its intensity at pleasure, 

 a thin wedge of coloured glass has been recommended, pre- 

 vented from acting as a prism by a similar wedge of plain glass 

 placed in contact with it the reverse way. A plain glass wedge 

 between two tinted ones of red and green, each of half the 

 angle of the colourless one, was used by the Astronomer Royal 

 for the eclipse in 1851. Such combinations must, of course, 

 be made to slide easily across the eye-hole, or be held in the 

 hand during observation. To attain the object of variable 

 intensity, the ancient plan of smoking a piece of glass succeeds 

 as well as far more expensive contrivances ; it is also said to 

 intercept heat much more completely than its hue might have 

 led us to expect ; probably in consequence of the absorptive 

 power of the carbon; a slip may be nicely graduated as to 

 depth by a little care in smoking, but will require to be pro- 

 tected from accident by another piece of clear glass placed 

 over it, and kept from touching it by interposed bits of paper. 

 In the preference of tint, however, another consideration must 

 be taken into account, which ought to influence our choice in 

 delicate observations. There is reason to believe that some of 

 the minuter solar details possess a decided colour, which would 

 be acted upon more in proportion than white light, by a screen 

 of such a hue as to neutralize their own. Delicate veils of a 

 ruddy cast, for example, such as have been noticed by Secchi, 

 might be rendered imperceptible by the non-transmission of 

 their light through green glass, or even a combination into 

 which it entered; while the general clearness of the rest of 

 the image would give no intimation that such a defalcation had 

 taken place ; and instances are on record where the remark- 

 able phenomena of a great solar eclipse have been consi- 

 derably modified from this cause. It would therefore be 

 advisable, when minute features are to be carefully scrutinized, 

 to be prepared with glasses of various tints. 



A strong reason for caution, however, when ordinary screen- 

 glasses are employed, exists in the fact that different telescopes 

 seem to have different foci for heat. Mr. Reade, in one in- 

 stance, found the burning effect much the strongest a little way 

 short of the solar focus, so that the calorific rays diverged, 

 while those of light emerged parallel from the eye-piece ; and 

 hence he recommended an eye-hole, like that of a Gregorian 

 reflector, between the eye-lens and the screen-glass, to inter- 

 cept the heat ; by which means he found that an aperture of 

 six inches could be used with safety. On the other hand, a 



* It is a cuiious fact that this mode of observation with two differently stained 

 glasses was anticipated, before the invention of the telescope, by Fabricius, in the 

 eolar eclipse of 1590. 



