OCTCBER 8, 1891] 



NAXURB 



555 



right angles to the axis of the lens, aad are expressid as frac- 

 tions of the focal V nglh. 



The abscissae are the inclicalions (ia degrees) of th« pencils to 

 the ax's 



Suppose, now, that the plate is placed at a distan;e e bchird 



Fij. 3. — Rapid Rectilinear. 



the principal focus, e being half the distance between the fic 

 for direct pencils of ihe nearest rnd distant objects. 



The worst defined point in the centre of the picture will then be 



represented as having a width ^^. ^ ncarlj', whi'e at the obliquity 



this width becomes - - — ^ ner.rly, accordin;^ to whether the 

 F CIS e* 



l;c.. c.— Triplet. 



image under consideration is that of the most distant or thp 

 nearest point. 



Hence, unless >» is small compared with e, the definition for 

 rays of obliquity 9 will be sensibly worse than ia "the centre of 

 the plate, and a reference to the curves fur j';, and ys shows at a 

 glance that this must be the case even at 10' for all the lenses 



unless the general standard of the definition is lowered by a 

 large value of e. 



A? an example of the use of the curves, let us take the rapid 

 rectilirear No, 6, and compare the definition at zo" obliquity- 

 wiih that at ihe centre, supposing that the nearest object Is at 

 I a distance of 25F. 



This gi^es e = 'ozY neaily, and at 20° ;> = - -023?,. 

 ys - + "016 F, hence we have as follows :— 



\ At 30° from axis 



This shows that while the nearest poinis at this obliquity are 

 represented by long ovals placed as if radiating from the axis, 

 the most distant poinis become similar but rather smaller ovals 

 with their long axes at right angles to the former, and that the 

 length of the ovals is about twice the diametir of the image 

 formed by ihe direct pencils. 



In the same way the definition, as far as it depends on 

 astigmatism and curvature of field, at any obl-quity may be 

 found for any lens for which _» and j',- are known. 



Lauriston Hall, September 9. A, Mallock. 



( To U continued. ) 



THE KOH-I-NUR—A CRITICISM. 



T^HE true history of the Kohi-Nur diamond, if it could be 

 ^ written, would be a singulaily interesting one. But the 

 historian would have a difficult task. The pages that I purpose 

 writing will be devoted to the criticism, possibly the refuting, of 

 some fallacies that hang round the .subject ; but they w ill not 

 deal with some other hi^torical difficulties that I have not space 

 even to indicate, but w hich do not belong to those poi tions of the 

 history for criticism on which the following pages are designed. 



The period in the history of the Kohi-Nur that has attracted 

 the notice of all modern writers on the diamond, and to a 

 degree, I think, somewhat beyond its importance, is the five or 

 ten minutes during which the French diamond-dealer, Tavemier, 

 held in his hand the most important of the Crown jewels of the 

 Emperor Aurungzebe. It was a great diamord, and the record 

 T<ivernier has handed down in his " Voyages," of its weight, its 

 form, and its history, will have to be critically dealt with. 



It may be at once staled that the disputable point regarding 

 this diamond is whether it was a certain ancient diamond of 

 fame in India, or one much larger than this ancient stone, that 

 had been found not very long before Tavemier was present at 

 the Court of Aurungzebe. For the larger slone I shall retain 

 the name of " the Great Mogul ' ; for the older and more famous 

 one the title of the Koh-i-Nur, Some hold that Tavernier saw 

 and handled ihe Koh-i-Nur ; others that his own story is correct, 

 and that it was the Great Mogul that he described. And I 

 should add that some, in addition to this latter view, believe the 

 Great Mogul ought to be called the Koh i-Nur. 



In order to clear the ground, I may say that while attaching 

 no very great importance to the question as to which of the two 

 first views is the correct one.^and I must add also, valuing at a 

 somewhat low estimate the historical or technical accuracy of 

 Tavernier's statements on this and many other matters— I, 

 some thiity-five years ago, came to the conclusion that the 

 diamond Tavernier saw was probably the Koh-iNur, and that 

 he muddled its history with the other and larger diamond that I 

 showed to have been probably at the time, in the keeping of 

 Shah Jahan, the capiive father of Aurungzebe. The merits of 

 the question will be discussed in their proper place ; but while 

 holding myself open to conviction if any nfew f.rguments can be 

 biought foiward against nry view, I may state that none yet 

 announced have .«hakcn that opinion. 



Until the fiiteenih century there appears to^ h»T6 been one 

 and only one very large diamond known in India or in the 

 wcrld. I might have said until the Sixteenlh century but 

 that there is a record of two and an unaulhenticaied rumour of a 

 third during that cenluiy, the; largest of, which, however, was 



NO. 1145, VOL. 44] 



