Camera Lucida . 
369 
tors there employed would be more generally understood. 
But those who are conversant with the science of optics 
will perceive the advantage that may be derived in this 
instance from prismatic 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. 3, represents the section of a solid prismatie piece 
of glass, within which both the reflections requisite are 
effected at the surfaces a &, b c, in such a manner that 
the ray f g, after being reflected first at g y and again at 
h , arrives at the eye in a direction h e at right angles to 
fs- 
There is another circumstance in this construction ne- 
cessary to be attended to, and which remains to be ex- 
plained. Where the reflection was produced by a piece 
of plain glass, it is obvious that any objects behind the 
glass (if sufficiently 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. 3. 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 pa- 
per are regulated by a small hole in a piece of brass, 
Which by moving on a centre at c, fig. 3, is capable of ad- 
justment to every inequality of light that is likely to occur 
