THE GREAT MELBOURNE TELESCOPE. 
133 
as, however, the Cassegrain is shorter, the space actually traversed, and therefore the 
chance of disturbance from this cause are as 1*12:1. Even this difference is nearly 
obviated by the lattice-tube. 
Fourthly, Newton advanced an objection which is often urged. When light is re- 
flected very obliquely from a body even of feeble reflecting power like glass, the reflection 
is almost total, its intensity decreases with the incidence, and when that is perpendicular, 
becomes very feeble. Now in the Cassegrain the incidence is nearly perpendicular, in 
the Newtonian at 45° ; and hence he inferred that the latter has most light. But we now 
know that metals reflect differently from glass, and that the intensity of their reflected 
light follows a more complicated law. It decreases down from the perpendicular inci- 
dence to one depending on the incidence of maximum polarization (which is different 
for different rays), and then increases till at 90° the reflection is total. In the researches 
to which I have' already referred, M. Jamin has given the intensity of red light from 
speculum metal, which when perpendicular =0*692, but for 45° is only 0*646 ; for the 
other rays the intensities are less, but follow the same law; so that the advantage is 
nearly ^ in favour of the Cassegrain. 
The other advantages are, first, the tube is shorter, therefore lighter, and less acted 
on by the wind ; second, the magnitude of the second image gives facility for micro- 
meter measures ; it is also flatter than in any other telescope ; third, the errors of the 
small speculum tend to correct those of the large ; and this is of some importance when 
they have to be repolished by persons not so skilful as the original maker. It is easier 
to figure the small mirror properly than the plane one of the Newtonian. Fourth, the 
greatest of all is the facility which it offers to the observer. The eyepiece is near the 
ground, and travels in a spherical surface of some 7-feet radius while the telescope 
sweeps the whole sky; the observer has to move but little, and the observing chair is 
light and easily managed. But with the Newtonian he must be at the upper end of the 
tube, which with a 4-feet looking to the zenith would be at least 37 feet above the 
ground ; and he must have an adequate apparatus to support him, the use of which re- 
quires much labour and is not altogether exempt from danger. This had most weight in 
deciding the Committee in favour of the Cassegrain, and the trials which have been 
made of the instrument since its completion fully justify their preference. 
In determining the relative powers of the specula and the lowest eyepiece, special 
formulae were in this case required, and those found in ordinary treatises of optics have 
the defect of assuming the distance of distinct vision infinite in respect of the focal 
lengths of the lenses. The conditions of the problem are three. 1. The opening in the 
large speculum should be as large as the aperture of the small one : no light is lost by 
this. 2. The opening of the field-lens of the lowest power should be of the same size 
to obtain the largest possible field of view. 3. The lowest power must be such that 
the eye can take in the whole emergent pencil. Call F, f f, and f" the focal lengths 
of the specula and the lenses of the Huyghenian, A and a the apertures of the specula, 
d and d' the distances of the first and second images from the small speculum, <p and n 
