Outstanding Colour. 185 



cretionary latitude, and a choice of evils, and different makers 

 do not prefer the same mode of correction. Glasses which are 

 technically said to be " over- corrected for colour/'' are gene- 

 rally thought the best. In these, the dark-blue rays which, 

 from their great refrangibility, naturally fall short of the 

 general focus, are projected beyond it by the concave flint, and 

 consequently form an external fringe to the image, which, from 

 their loss, is slightly tinged with the complimentary reddish- 

 yellow ;* but this over- correction may be carried too far, pro- 

 ducing a needless excess of blue, as is said to be the case 

 with the Munich object-glasses. Sir John Herschel has said, 

 that the whitest pencil is produced by uniting the brighter 

 red bordering on orange, and the most vivid blue, where it 

 begins to pass into green. Glasses thus corrected show a 

 purple or lilac fringe round a white object within the focus, 

 and a green one without it, and " to go beyond this point, " he 

 adds, " with the ordinary materials, seems hopeless." In the 

 various forms that we have described, the defect uniformly 

 exists. It may be somewhat reduced in more than one way : 

 in the triple achromatic (and by parity of reason it might be 

 so in the dialyte) by employing a third kind of glass, called 

 Savoy plate ; in the peculiar form of double object-glass, pro- 

 posed by Gauss, and modified by Steinheil (Intellectual Ob- 

 server, hi. 148), by means of a slight separation of the plate 

 and flint lenses : Barlow had hoped- to remove it in his con- 

 struction, but was disappointed; quadruple object-glasses 

 have been produced by Steinheil, of Munich, arid Grubb, of 

 Dublin, f in which it is reduced, but with a serious loss of 

 light by reflection, unless the inner surfaces are cemented 

 together, which is considered objectionable by the best 

 opticians. To one man alone has it been given to devise and 

 accomplish the complete removal of this great defect, and he has 

 never yet received that general acknowledgment of his great 

 ingenuity and skill, which was his unquestioned due. To- 

 wards the close of the last century, Dr. Robert Blair, Profes- 

 sor of Practical Astronomy in the University of Edinburgh, 

 carefully investigated the subject, and ascertained the exist- 

 ence of transparent and colourless fluids, whose separate action 

 upon the green ray (which may be considered the standard of 

 deviation) were respectively greater, and less, than that of 



* This should he carefully home in mind in examining the colours of stars. 

 Struve I. found that the higher powers of the Dorpat telescope (532 to 848) gave 

 them a yellowish tinge. 



t The observatory at Armagh is provided with one of these instruments of 

 seven inches aperture, with the unusually short focal length of sixty-eight inches, 

 which is probably rendered practicable by the double number of curved surfaces, 

 each, therefore, only half as deep as in the ordinary form. (These are Sir J. 

 Herschel' s data, but I am informed that the aperture is really eight inches.) 



