296 Solar Observation. 



of the field. To avoid this, tlie late Mr. Lawson, of Bath, 

 who was the possessor of a very fine 7-inch achromatic by 

 Dollond, presented by him, at the close of his life, to the 

 Greenwich Naval School, introduced the screen between the 

 object-glass and the focus, very near the latter, in which posi- 

 tion, however, it frequently was cracked by the heat. This is, 

 indeed, an accident to which these glasses are often liable. 

 The most experienced of solar observers, Schwabe, speaks of 

 it, though in his case the screen was probably placed, as usual, 

 in the exterior brass cap ; and he remarked that the occurrence 

 took place most commonly in years, such as 1833 and 1843, 

 when few spots were visible. In some measure this might 

 certainly be due to the lower temperature of the spots- them- 

 selves — a curious fact, which has been fully established by 

 Secchij but should their relative area be considered too 

 small to produce such a result, it would tend at any rate to 

 show that, contrary to the opinion of Sir W. Herschel, their 

 development was concurrent with diminished energy in the 

 calorific influence of the sun. 



As we have to deal with heat as well as light, it is by no 

 means immaterial by what means the darkening process is 

 effected. It is generally known that the rays of heat are dis- 

 tinct from those of light, and being less refrangible than the 

 latter, are co-incident for the most part with the red end of 

 the spectrum, extending even considerably beyond its ordinary 

 limits. Glass, therefore, which freely transmits rays of that 

 colour, being equally permeable by the rays of heat, is pecu- 

 liarly ill adapted lor a screen ; its frequent employment in the 

 solar caps of the older telescopes may probably have been 

 owing to the superior readiness with which it could be procured, 

 of sufficient depth, transparency, and uniformity of tint; but 

 its effect was distressing to the eye. Green would be far pre- 

 ferable, as intercepting the heat, but it is difficult to obtain it 

 of a binge si illieiently powerful to subdue the excessive light. 

 Deep yellow lias also been used, but nothing seems preferable 

 to a dark bluish-grey, or neutral tint, which gives a beautiful 

 and comparatively cool image. Combinations of colour have 

 been found very effeotim Since whito light is composed of 

 what artists call tho throe primary colours — red, yellow, and 

 blue, and the two latter form green, it is obvious that a combi- 

 natiou of red and green, provided the tints were carefully 

 balanced in quality and intensity, would transmit whito light, 

 with very little heal, tho calorific r;iys being intercepted by the 

 green glass ; and such screens are said to bo very pleasant. 

 Sir J. Herschel speaks highly of cobalt blue (tho colour of 

 finger-glasses) interposed between two thicknesses of green, 



