132 
DE. T. E. EOBINSON AND ME. T. GEUBB’S DESCEIPTION OF 
6-feet, found that it only gave 0 # 67. Part of this deficiency may arise from its mole- 
cular condition, but more arises from its being partly transparent to the more refran- 
gible rays of the spectrum, which it transmits so freely that Foucault proposed and 
used this silvered glass as a shade in solar observations ; for this reason also the images 
will be tinged with red. The silver film would tarnish faster than the speculum metal ; 
and though on a small scale it is easily renewed, the manipulation of a speculum of 
such size, and probably of 5 cwt., would present considerable difficulty. Nor is it 
impossible that the film might break up under considerable changes of temperature, 
such as occur at Melbourne, or he spotted by rain or dew. 
Y. The last matter which requires explanation is the Committee’s decision that the 
telescope should be of the Cassegrain construction. This was not made without con- 
siderable discussion ; for very few have ever been made on this plan, and only two of 
the Committee had any practical acquaintance with them. Lord Posse made one of 
18 inches aperture; but the specula were not properly proportioned, and he used it but 
little. I had a good deal of experience with a 15-inch one, and was perfectly satisfied 
with its performance on such stars as s Arietes, r\ Coronse, and £ Herculis when closest ; 
but still the step from 15 to 48 inches was an adventurous one. The hostility of Newton 
to this telescope has probably created a prejudice against it; and it has objectionable 
points, which, however, are, I think, more than compensated by certain advantages which 
belong to it. Its greatest defect is the difficulty of obtaining a low magnifying-power. 
As the image formed by the great speculum is magnified by the small one, from five to 
six times, the eyepiece must be as much weaker than in the Newtonian; and as the 
lowest power must be such, to obtain the whole effect, that the eye can take in the whole 
pencil, which according to Sir W. Herschel may not exceed two-tenths of an inch 
diameter, we come to rather formidable dimensions for it. With a 4-feet speculum this 
lowest power =240, and the Huyglienian eyepiece which gives it is nearly 9 inches 
diameter and 12 long. The glass for its lenses, though it need not be so faultless as for 
an object-glass, must be of good quality, and therefore costly. This is not of so much 
importance as the thickness of the glass in the lenses, which, as I have already shown, 
diminishes the light, and this thickness cannot be diminished without a corresponding 
decrease of the field of view *. In a telescope of this size it might perhaps be desirable 
to employ a triple eyepiece, possessing the Huyghenian properties of achromatism and 
equal flexure of the pencils ; such a one, in which the distance of the second and third 
lenses =-§ the focal length of the third, would have only 0 - 6 the thickness of glass, and 
this difference might probably be more than equivalent to the extra pair of reflections. 
This eyepiece has a much flatter field and is as sharp as the Huyghenian. 
Secondly, the small mirror is something larger than its rival’s, and therefore inter- 
cepts more light; the difference is only a fiftieth of the whole, and it must be re- 
membered that the central rays are not those which give the best vision. 
Thirdly, the rays traverse the tube thrice, but in the other a little more than twice ; 
* But as to this, see Appendix. 
