438 



Dr. Royston-Pigott on 



[June 19, 



the flint glass and another the crown, are furnished with long rods 

 terminating with similar screws with divided heads. 



If on testing the telescope severely by applying a microscope accu- 

 rately adjusted to the solar disk, diminished sufficiently by a distant 

 inverted objective in front of a heliostat described, the telescope object- 

 glasses be overcorrected, the separable lenses of the telescopic object- 

 glass will require closing together slightly by moving the screw-heads 

 through equal spaces ; and, vice versa, slight residuary aberrations will be also 

 affected by changes in the compensating eyepiece. The tubes should be 

 ground (in the manner of making levels) truly to preserve the centering 

 of the objective lenses. 



Another form of correction is the concave achromatic lens, now called 

 the Barlow lens. If this lens be formed of an overcorrected combina- 

 tion, its effect on the fiual aberration will vary as it receives a larger or 

 smaller pencil, i. e. as its position between the eyepiece and objective of 

 a telescope or microscope is made to vary. The traversing principle 

 here advocated has therefore several important advantages worthy of 

 investigation, and may possibly form a correction for changeable refrac- 

 ting powers of the atmosphere. 



In the compensating eyepiece a large visual pencil is substituted for a 

 minute one and the same ease and comfort is obtained with it as if the 

 linear aperture of the telescope were greatly enlarged. An ocular of 

 one-inch focus may be used instead of another ten times as deep. 



Slight' changes in the over- or undercorrection of the telescope 

 depending on the changeable density of the atmosphere may also pro- 

 bably be compensated by over- or undercorrecting this eyepiece. 



Moreover the special aberrations of the eye-glasses may be ameliorated 

 by a slight change of form and an alteration in the interval between 

 them. . 



JSote — June 25, 1873. 



The scarcity of sunshine this year has prevented me carrying out a 

 complete series of experiments in telescopes. My excellent 8^-inch 

 Browniug equatorial (one of his very best) shows little spherical aberra- 

 tion. Two very good achromatic telescopes with the ordinary eyepieces 

 gave imperfect jet-black rings, few in number, with the ordinary Huyghe- 

 nians ; but with the compensated eyepiece I counted six intensely black 

 rings. The definition was excellent. 



I trust observers with more convenient appliances and range of pros- 

 pect will be induced to try a series of experiments on correcting residuary 

 aberration in their telescopic object-glasses by the methods here laid 

 down, and the writer will at any time be honoured by exhibiting to them 

 some of the phenomena here described, as already done to Dr. Gladstone, 

 F.E.S., and Mr. Beaumont, F.E.S. 



