[122 rERSPECTIVE INSTRUMEMT, EtLIPTIC INSTRUMEMT* 



but if we add a further quantity, the lime is again decom- 

 pofed, and the lime-water becomes again perfectly clear, 

 I am. Sir, 



Your moft obedient fervant, 



RA. THICKNESSE. 



XIII. 



Defcription of an Injirument for drawing in true PerfpeStive 

 from Nature, and of another of confiderahle Simplicity and 

 Cheapnef^i for delineating Ovals, In a Letter from a CorrC' 

 fpondent, R. B. 



To Mr. NICHOLSONf ^ 

 SIR, ^ 



introduaion. J\s I obferre that you are willing, in your capacity of Jour- 

 nalift, to lay before the public any (ketch or outline of inven- 

 tion that may promife to be ufeful, whether in its ultimate 

 flate of improvement or not, I am encouraged now and then 

 to fend my thoughts, queries, obfervations, or news, as they 

 may occur. The following inftruments are offered to your 

 notice, in hopes they may appear in your excellent col- 

 ledion. 

 Inftrument for Fig, 1, Plate VIU, is a Iketch of an inftrument for per- 

 drawmg in per- fpe^c^iyp^ made fome years ago by Dolland, and of which I 

 know not the inventor. A telefcope or camera is fufpended 

 vertically on a frame by an univerfal joint or jimbals. Hori- 

 zontal rays A, are diredted down the tube by a plane mirror 

 B, and are again rendered horizontal, and turned to the eye 

 through a fide hole in the tube, by another mirror C. At the 

 lower end is a pencil E Aiding in a well-fitted focket, and 

 preffed gently downwards by a weight or fpring ; or ftill better, 

 by the hand only. The refult or ufe is, that while the images 

 are in fucceffion brought into apparent contadl with a point 

 in the field of view, the pencil may be employed in tracing 

 them in true perfpediive upon the table beneath *, 



* There is an omiflion of the grey or rough glafs, if the drawing 

 be meant for a camera} or of the eye-piece, if it be a telefcope. 

 The firft focal convergence muft be made in thcfe, and not at the 

 eye— N. 



5 Fig^ 



