26 REPORT—1874. 
planes of polarization of the rays proceeding from the four images may be deter- 
mined by introducing a Nicol’s prism into the tube, to which the eye is applied. 
On the Construction of large Nicol’s Prisms*, By W. Lavp; F. RAS. 
On the Construction of a perfectly Achromatic Telescopert. 
By Professor G. G. Sroxus, M.A., Sec. LS. 
At the Meeting of the Association at Edinburgh in 1871, it was stated that it 
was in contemplation actually to construct a telescope, by means of disks of glass 
prepared by the late Mr. Vernon Harcourt, which should be achromatic as to 
secondary as well as primary dispersion. This intention was subsequently carried 
out, and the telescope, which was constructed by Mr. Howard Grubb, was now 
exhibited to the Section. The original intention was to construct the objective 
of a phosphatic glass containing a suitable percentage of titanic acid, achromatized 
by a glass of terborate of lead{. As the curvature of the convex lens would be 
rather severe if the whole convex power were thrown into a single lens, it was 
intended to use two lenses of this glass, one in front and one behind, with the 
concave terborate of lead placed between them. It was found that, provided not 
more than about 3 of the convex power were thrown behind, the adjacent surfaces 
might be made to fit, consistently with the condition of destroying the spherical as 
well as the chromatic aberration. This would render it possible to cement the 
glasses, and thereby protect the terborate, which was rather liable to tarnish. 
At the time of Mr. Harcourt’s death, two disks of the titanic glass had been 
prepared, which it was hoped would be good enough for employment, as also two 
disks of terborate. These were placed in Mr. Grubb’s hands. On polishing, one of 
the titanic disks was found to be too badly striated to be employed; the other was 
pretty fair. As it would have required a rather severe curvature of the first 
surface, and an unusual convexity of the last, to throw the whole convex power 
into the first lens, using a mere shell of crown glass behind to protect the 
terborate, Prof. Stokes thought it more prudent to throw about } of the whole 
convex power into the third or crown-glass lens, though at the sacrifice of an 
absolute destruction of secondary dispersion, which by this change from the original 
design might be expected to be just barely perceptible. Of the terborate disks, the 
less striated happened to be slightly muddy, from some accident in the preparation; 
ie as this signified less than the strie, Mr. Grubb deemed it better to employ this 
disk, 
The telescope exhibited to the Meeting was of about 23 inches aperture and 
28 inches focal length, and was provided with an objective of the ordinary kind, 
by which the other could be replaced, for contrasting the performance. When 
the telescope was turned on to a chimney seen against the sky or other suitable 
object, and half the object-glass covered by a screen with its edge parallel to the 
edges of the object, in the case of the ordinary objective vivid green and purple 
were seen about the two edges, whereas with the Harcourt objective there was 
barely any perceptible colour. It was not, of course, to be expected that the 
performance of the telescope should be good, on account of the difficulty of 
preparing glass free from strie, but it was quite sufficient to show the possibility of 
destroying the secondary colour, which was the object of the construction. 
On a Form of Spottiswoode’s Triple Combination of Double-Image Prisms and 
Quartz Plates applied to the Table Polariscope. By 8. C. Tistey. 
This instrument consists of an eyepiece substituted for the usual Nicol prism 
analyzer of the ordinary table polariscope. It contains five cells capable of being 
* Published in ‘ Nature,’ vol. x. p.451 (Oct. 1, 1874). + Ibid. p. 431 (Sept. 24, 1874). 
_{ The percentage of titanic acid was so chosen that there should be no irrationality of 
dispersion between the titanic glass and the terborate. 
