526 
LORD OX MAN TOWN ON THE REFLECTING TELESCOPE. 
and M(Edler. A much higher magnifying power can also be used with decided ad- 
vantage than they say has hitherto been practicable, viz. 300* : with a power of 600, 
and sometimes with a power of 900, many details are brought out, not visible with 
lower powers. 
Were it practicable to construct a rectangular prism, reflecting as accurately as a 
flat metal, notwithstanding the thickness of the glass, there should, I think, be a con- 
siderable saving of light : I have not as yet had time to try the experiment, but I 
think it worth a fair trial. Such prisms, however, do not seem to have performed as 
well as might have been expected, perhaps owing to imperfect workmanship. 
A still greater accession of light might be obtained without sacrifice of defining 
power, by using the Herschellian construction, were it possible to discover some 
means of working approximately the surface of accurate reflexion for oblique rays. 
I have recently tried a few experiments on a small scale on this subject, and am 
disposed to think the task is not hopeless ; but a course of experiments on a large 
scale would be required to afford a decisive result. The question is not whether 
such a figure could be worked as accurately as a spherical one ; of that I have 
no hope ; but whether, in practice, such a degree of accuracy might not be ob- 
tained, that the mirror would define decidedly better than if it had been spherical, 
and as well, or nearly so, as when worked to the best parabolic figure that can be exe- 
cuted, and used as a Newtonian. The principle on which I have proceeded in these 
experiments was simply to consider the reflecting surface sought, as a portion of a 
paraboloid whose axis coincided with the side of the tube, the eye-piece of course 
placed on the axis ; and in endeavouring to work that figure, I have had recourse to 
no other expedient than an adjustment of the motions with respect to the position of 
the speculum, guided by the same view of the subject which directed the attempts to 
work the paraboloid for the Newtonian, varied merely to suit the altered circum- 
stances ; but in the present imperfect state of these experiments it would be waste of 
time to enter more into particulars. I have mentioned the subject merely for the 
purpose of directing the attention of others to it. 
Sir William Hamilton, in his paper in the Transactions of the Royal Irish Academy 
for 1828, on Systems of Rays, considers the surface as the envelope of an ellipsoid of 
revolution, having a constant axis, but a variable eccentricity, moved in such a man- 
ner, that while one focus traverses in all directions the surface which cuts the inci- 
dent rays perpendicularly, the other focus remains fixed at the point through which 
all the reflected rays are to pass. I have not, however, discovered any practical means 
of availing myself of his very original mode of treating the subject. 
To conclude, I think I may state as the results of all these experiments, that specula 
can be made to act effectively, cast of the finest speculum metal, in separate portions, 
* Bis jetzt ist eine 300 malige Vergrosserung die stdrkste die man mit verhaltnissmassigem Erfolge auf den 
Mond anwenden konnte, p. 5, note. 
