Description of the Camera Lncida. 345 



may be wanted lo suit an eye that is long- or short-sighted. 

 Those persons, however, v.liose sight is nearly perfect, may 

 at pleasure use either of the glasses. 



The instrument represented in that figure differs moreover 

 in other respects from the foregoing, which I have chosen 

 to describe first, because the action of the reflectors there 

 employed would be more generally understood. But those 

 who are conversant w ith the science of optics will perceive 

 the advantage that may be derived in this instance from pris^ 

 niatic reflection ; for, when a ray of light has entered a solid 

 piece of glass, and fills from within upon any surface at ari 

 inclination of onlv 2-2 or 23 degrees, as above supposed, the 

 refractive power of the glass is such as to suffer none of that 

 light to pass out, and the surface becomes in this case the 

 most brilliant reflector that can be employed. 



Fig. 2. represents the section of a solid prismatic piece of 

 glass, within which both tlie reflections requisite are efTtcted 

 at ibe surfaces a/', be, in such a manner that the ray fg, 

 after being reflected first at g and afterwards at II, arrives at 

 the eye in a direction he, at right angles io fg. 



There is another circumstance in this construction neces- 

 sary to be attended to, and wliich remains to Ije explained. 

 Where the reflection was produced by a piece of plain glass, 

 it is obvious that any objects behind the glass (if sufficiently 

 illuminaicd) nrigl:t be seen through the glass as well as the 

 reflected image. But when the [)rismatic reflector is em- 

 ployed, since no light can be transniiltcd directlv through 

 it, the eye must be so placed that onlv a part of its pupil 

 may be intercepttd by the edge of the prism, as at e, ds,. 2, 

 The distant objects will then be seen by this portion of the 

 eye, while the paper and pencil arc seen pa^t the edsie of the 

 priim by the remainder of the pupil. 



In order to avoid inconvenience that might arise from un- 

 intentional motion of the eye, the relative quantities of light 

 to be received from the object and from the paper are regu- 

 lated by a small hole in a piece of bra-s, which, by movin<»' 

 on a centre at r, fig. 3, is capable of adjustment lo every 

 ine'|uality of light that is likely to occur. 



Since the size of the whole instrument, IVom being so 



near 



