Solar Observation. 295 



injurious. The object-glass of a telescope is, in fact, a burn- 

 ing-glass, and though its comparatively long focus, from the 

 enlargement of the image, disqualifies it from producing the 

 most powerful heating effect in proportion to its area, yet its 

 energy in this way is not to be trifled with. A remarkable 

 instance is given by Secchi of the activity of a 9~ inch ob- 

 ject-glass in the pure sky of Rome. Without any further 

 concentration of the cone of rays than was due to the field 

 lens of an eye-piece, a piece of the whitest paper exploded in 

 the focus instantaneously like gunpowder, and 3 grammes 

 (= 46*3 grains) of lead were melted in less than two seconds. 

 Various schemes have been devised at different times to obviate 

 the danger from this source. The most natural one, that of 

 contracting the aperture of the telescope to very small dimen- 

 sions, is not so successful as might perhaps have been expected, 

 since, for reasons which involve a knowledge of mathematical 

 optics, the focal image becomes less defined in proportion to 

 the acuteness of the angle at which the intersection of the 

 rays takes place. Herschel I., therefore, in older times, and 

 Dawes in these, not to mention other observers, have pro- 

 nounced in favour of using the largest available aperture ; and 

 the latter has remarked that the solar phenomena, " when 

 carefully scrutinized with large apertures and high powers 

 under suitable atmospheric circumstances, are so wonderfully 

 different in their appearance from those presented by the dimi- 

 nished apertures formerly and necessarily in use, that it would 

 not be very surprising if some observers, unaware of what had 

 previously been seen and described, should imagine that the 

 phasnomena revealed by their newly acquired and powerful 

 telescopes were really new discoveries." The excessive and 

 perilous light and its attendant heat must therefore be all ad- 

 mitted first, and neutralized afterwards, as best we can. This 

 could not be well done by interposing any screen of dark- 

 coloured glass in front of the object-glass, since it would be 

 difficult to find a sufficiently homogeneous piece of the required 

 diameter, or to work it to plane and parallel surfaces with due 

 correctness.* It has commonly been introduced behind the 

 eye-piece, and close to the eye, in which position its very small 

 dimensions exempt it from the disadvantages which have been 

 mentioned ; but even there it is not pleasant in use, as prevent- 

 ing the eye from coming near enough to command the whole 



* I have somewhere met with a suggestion, but do not now recollect where, as 

 to the construction of a solar telescope by employing as an object-glass a single 

 lens of deep-coloured glass ; this, transmitting only rays of nearly the same re- 

 frangibility, would bo sufficiently achromatic, but the uncompensated spherical 

 aberration would render rather a long focus desirable. The idea is iugenious, and 

 might be worth a trial. 



