174 © S. Hastings—Color Correction of Double Objectives. 
IV. Practical tests of the value of the foregoing theory. 
There are two evident methods for determining the value of 
the results which we have obtained; one is to construct a tel- 
escope according to the principles developed in the theory and 
study its performance, and the other is to find how the practice 
of the most approved makers agrees with that as founded upon 
the theory. Both of these tests I have applied with thorough- 
ness and will here give the results in the order named. 
Three object glasses, involving the use of six varieties of 
glass, have been constructed according to the principles devel- 
oped above. The first was of 4:1 inches aperture and 53 inches 
focal length, with the flint in advance of the crown. e mate- 
rials of which this objective are made are the flint glass of Feil, 
No. 1237, and the crown glass of the same maker, No. 1219. 
The focal length was purposely made small so as to yield the 
severer test. 
The second objective has a clear aperture of 6} inches and a 
focal length of 91 inches. The crown lens is in advance and 
the curves are such as satisfy, for a first approximation, the 
conditions proposed by Sir Jobn Herschel. ‘This form, though 
e 
ground very exactly according to the values yielded by calcula- 
tion and the objective ,finished before testing. An extremely 
delicate spherometer was employed to determine the radii. As 
all the objectives are of the highest excellence, the usefulness 
of the theory may be regarded as demonstrated. 
For the second test proposed, we possess the results of the 
studies of two distinguished spectroscopists on the character of 
the color correction of several large telescopes. To these I can 
add two more. 
inch rh 2 
* The optical properties of these glasses as well as those of the 6 } 
tive are given in a paper entitled, On the influence of Temperature on the Optica 
Constants of Glass, This Jour., vol. xv, pp. 269-275, 1878. 
