CAMERA LUCIDA. 



3 



to describe first, because the action of the reflectors there strament. 

 employed would be more generally understood. But those fl "J^ " lc re ° 

 who are conversant with the science of optics will perceive 

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

 matic reflection ; for when a ray of light has entered a solid 

 piece of glass, and falls from within upon any surface, at 

 an inclination of only twenty-two or twenty-three 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 Figure of the 

 glass, within which both the reflections requisite are effected P rism - 

 at the surfaces a b, b c, in such a manner that the ray f g 9 

 after being reflected first at g, and again at h, arrives at the 

 eye in a direction h e at right angles tofg. 



There is another circumstance in this construction neces- Remedy for 

 sary to be attended to, and which remains to be explained, ^^n^sion of 

 Where the refleetion was produced by a piece of plain glass, direct light 

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

 illuminated) might be seen through the glass as well 

 as the reflected image. But when the prismatic reflector is 

 employed, since no light can be transmitted directly through 

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

 may be intercepted by the edge of the prism, as at e Fig. 2. 

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

 eye, while the paper and pencil are seen past the edge of 

 the prism by the remainder of the pupil. 



In order to avoid inconvenience that might arise from 

 unintentional motion of the eye, the relative quantities of 

 light to be received from the object, and from the paper 

 are regulated by a small hole in a piece of brass, which by 

 moving on a center at c, fig. 3. is capable of adjustment to 

 every inequality of light that is likely to occur. 



Since the size of the whole instrument, from being so near The instrument 

 the eye, does not require to be large, I have on many ac- is of small di- 

 counts preferred the smallest size that could be executed 

 with correctness, and have had it constructed on such a 

 scale, that the lenses are only | of an inch in diameter. 



Though the original design, and principal use of this in- It may be used 



