IVbllastoii' s Periscopic Spectacles. 273 



bucket : in the centre of these pieces is placed the small 

 iron standard M, on which is a collar with four arms N, 

 made to move up and down : above the collar is a smaJi 

 brass pullev in the mortice O, in the upper part of the 

 standard, and a loop to which the well-rope P is made fast ; 

 to one of the arms of the collar is tied a cord, which passes 

 over the brass pulley above, and then down to the lip of the 

 valve ; so that the cross L, being pressed against the bars B, 

 is forced downwards, the cord drawn up^ and the valve 

 opened. 



XLV. An Examination of Dr. Wollaston's Experiment 

 on his Periscopic Spectacles. By Mr. William Jones, 

 F.Am. P. S. 



To Mr. Tilloch. 



SIR, 



J. HE inferences that Dr. Wollaston has thought it best to 

 publish in your last month's magazine, instead of a direct 

 reply to my refutation of his new principle of spectacle 

 glasses, are of themselves sufficient to convince any impar- 

 Tial person of the validity of the objections advanced by me 

 in vour preceding number; and, notwithstanding an extra- 

 ordinary experiment he has therein related, as made only 

 by himself, I should not have thought it requisite to trouble 

 your readers again, but for the unfounded imputation he 

 has declared against me, that of having, by an experiment, 

 deceived myself. I trust, sir, I may be allowed, in con- 

 tradiction to this, to observe, that after more than twenty 

 years experience in the practice of my profession, such as 

 daily administering to decayed vision, and employment in 

 the construction of all kinds of optical instruments, I should 

 not be acquainted with the various properties of lenses, 

 singly or combined, and especially of so simple and well 

 known a form of lens as adopted by him, is an idea that I 

 am confident he cannot impress upon the mind ot the pub- 

 lic. I suggested no new experiment, nor was any one 

 wanting; the definitive laws I adduced were contained in 

 the works of the best writers on optics, and were sufficient 

 to evince the want of originality and improvement of his 

 meniscus-shaped lens. In respect to the experiment by 

 which he attempts to enforce a proof of an advantage in his 

 spectacles, its value will be known by the following account 

 of a repetition of it. I am i)0ssesied of a pair oi his peri- 

 VoL.XVid.No. 71. S scopic 



