1864. | Tfurscuun on the Solar Spots. 221 
degree of perfection is procured, which may be impressed photo- 
graphically or delineated manually. The former is the mode practised 
at the Kew Observatory by Mr. De La Rue, and we believe by most other 
helio-photographers : the latter is understood to be the origin of those 
exquisite drawings laid before the Royal Astronomical Society by 
Mr. Howlitt. One improvement only seems yet wanting to render 
either of these modes of procedure as satisfactory as actual vision 
through the telescope—viz. in the place of the ordinary telescopic 
eye-piece to substitute an achromatic and aplanatic object-glass of 
short focus and sufficiently large aperture, having the radii of the 
surfaces of its two lenses calculated on the principles laid down in my 
paper (‘ Phil. Trans.,’ 1821) for the construction of such an object- 
glass. The radii so calculated afford a lens, aplanatic not merely for 
parallel rays, but for all distances of the radiant point, so that when 
inverted, or placed with its flint lens towards the light, and used as a 
microscope, it produces neither colour nor spherical aberration, and is 
thus excellently fitted for projecting a magnified image, perfect not 
only as to the form, but as to the colour, of the spots, and on a scale of 
any desired enlargement, by a mere change of focus and corresponding 
alteration of the screen’s distance. 
When the telescope is used as a telescope, the great brightness and 
intense heat of the sun require to be subdued, to make observation 
possible. It is a common mistake to suppose that this can be done by 
merely contracting the aperture of the object-glass by a circular dia- 
phragm placed before it. In practice this is fatal to distinct vision. 
Ceteris paribus, in telescopic vision, the sharpness of definition is in the 
direct ratio of the angle (within moderate limits) which the object-glass 
subtends at its focus. Any attempt to evade this law by stopping out 
the light by concentric annuli will be found to issue in worse confusion. 
To use the full aperture of the telescope is of paramount necessity either 
in viewing the sun or planets. If the extinction of the light is effected 
by coloured giasses, the best combinations I have yet found are: Ist, 
that of two plane glasses of a shade between brown and violet, with one 
of a grass-green hue interposed: or 2nd, of two green glasses, with a 
blue one coloured by cobalt between them. These allow scarcely any 
rays of the spectrum to pass but the yellow and less refrangible green ; 
and they cut off almost all the heat. The perfection of vision is at- 
tained by using only the extreme red rays; but glasses which transmit 
these cannot be used on account of the heat they allow to pass. What-_ 
ever combination of glasses be used, they are, however, apt to crack and 
fly to pieces through the heat which they do intercept. Hence the ne- 
cessity of either limiting the field of view by a metal screen with a 
small hole in the focus of the object-glass, as recommended for trial 
by Wilson, in 1774, and as practised with excellent effect by Mr. 
Dawes ; or of some construction of the telescope itself, which, in the 
act of forming the image, shall suppress a very large percentage of 
the whole incident light, without preference of colour. Such is the 
object of the ‘“Helioscope” described in my “Cape Observations,” * 
* (1847), page 436. 
