6 Mr. Howarp Gruss on Great Telescopes of the Future. 
B 1.—Absence of Secondary Spectrum. 
Owing to the irrationality of spectra given by the flint and crown glasses at 
present obtainable, there remains always in the best corrected objective a little 
fringe of outstanding colour, technically called the “secondary spectrum.” It 
is impossible to avoid this defect unless the glass manufacturers succeed in making 
glass of different optical qualities to that at present in use; “Gauss” in theory 
and “ Steinhiel” in practice tried to unite the central rays with those of a zone 
near the circumference, but the form of the resulting objective became very 
impracticable, and the result was no better than the less complex forms. Professor 
Stokes and the late Mr. Vernon Harcourt tried a number of experiments with 
the view of obtaining two kinds of glass with rational, or nearly rational, spectra. 
These glasses I worked into an objective for Professor Stokes. The result was 
successful so far as the obtaining of specimens of phosphatic glass with rational 
spectra ; but phosphatic glass is almost unworkable, and when the experiment was 
tried on a siliceous glass it failed. Some alleviation of this secondary spectrum 
can be obtained by using a triple objective, but with, of course, a corresponding 
loss of light. 
It may, therefore, be assumed that at present there is no good prospect of any 
large objective being made without secondary spectrum, and, unfortunately, this 
increases very rapidly with increase of aperture of objective. 
B 2.—Applicability of Reflector for Physical Work. 
This is the great point of advantage which the Reflector possesses over the 
Refractor. The extraordinary strides made in Physical Astronomy of late years, 
and its still increasing importance, places this point of superiority in the very 
first rank for consideration, and this one point alone in many cases actually neces- 
sitates the adoption of the Reflector. 
Tt was for this reason that, when furnishing the 15-inch Refractor for Dr. Huggin’s 
use, the Royal Society, with admirable foresight, ordered me to adapt an 18-inch 
Reflector to the same mounting, and it is with this Reflector that Dr. Huggins has 
within the last few months obtained the photograph of the spectrum of a Lyre, 
with which he has astonished the astronomical world, and which it would not have 
been possible to obtain with a Refractor. 
It is needless to go through all the various points in which the Reflector has the 
advantage as regards physical work; they are well known, and are fully treated of 
elsewhere ; suffice it to say that in all the branches of stellar spectroscopy, photo- 
graphy, and in thermometric experiments, the Reflector possesses great advantages, 
and in some cases the experiments can only be carried on by their use ; and finally, 
be it remembered, that these are just the very experiments to which a monster 
telescope should be, with greatest advantage, devoted. 
