436 Queries regarding Dr. Young's Optometer. 



dish and Scottish deposits (Geological Proceedings, vol. iii. 

 p. 119*). 



Owing to the thick alluvial cover, the precise extent of this 

 shelly deposit cannot be determined : it is certainly known to 

 occupy several acres, but it may extend much farther. Traces 

 of a clay deposit with shells have been noticed at several places 

 on the northern shores of the bay, at heights reaching from 

 near the sea-level to sixty feet. They have been passed 

 through in well-borings, but have yielded only a few frag- 

 ments. The intended cuttings on two projected lines of rail- 

 way will give complete sections of the superficial clay beds, 

 and it is to be hoped, will furnish some interesting facts. 



LXII. Queries regarding Dr. Young's Optometer. 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 

 TJOWEVER eminent the abilities of the late Dr. Young, 

 •*--^ he certainly did not study the art of writing in such a 

 style, that not only he might possibly he understood by those of 

 his readers who comprehended the subject nearly as well as 

 himself, but that he could not possibly be inisunder stood by any 

 one of ordinary capacity and attention — an invaluable art, too 

 little attended to by English philosophers. 



I should feel obliged to any of your readers, who understand 

 Dr. Young's optometer, as described in his Natural Philoso- 

 phy, vol. ii. p. 576, and the scales of that instrument as re- 

 presented in fig. 7^ of plate 9, who would give, through your 

 pages, a plain and perspicuous explanation of them. 



The following queries will direct attention to some of the 

 particulai's on which I wish information: — 



1. Dr. Young says, "In order to adapt the instrument to 

 the use of presbyopic eyes, the other end must be furnished 

 with a lens." Now, which end is suited for examining the 

 focal distance of normal eyes, the one with or that without the 

 lens? 



2. One line of the scale is a scale of inches; another is di- 

 vided according to a "table here calculated, by means of which, 

 not only diverging, but also parallel and converging rays from 

 the lens are referred to their virtual focus." What is the cal= 

 culation here referred to ? In the series of numbers, the mark 

 of infinity occurs, while some of the numbers have the nega- 

 tive sign before them, and others after them ; what does this 

 signify ? 



* Inserted also in Fhil. Mag., S. 3. vol. xv, p. 400, 



