Observations on a new Kind of Spectacles. 89, 
conclusion that the form should be (LombZe) convex with- 
out and concave within, so that rays’ coming to the eye 
would pass nearly at right angles to the surface of the glass 
an all directions. These glasses were called by the inventor 
Periscopic, and the exclusive sale of them was secured to 
Messrs. Dollond by patent. 
"My attention having been some time since drawn to this 
subject by an article in Nicholson’s Journal, 1 proposed a 
trial of them to my friend M. Cauchoix, well known as a 
skilful optician in general, and more particularly by the 
large achromatic lenses which he has lately made of flint= 
glass manufactured in France by M. Dartigues. I re- 
quested his opinion on the subject; for, though our theory 
should direct the artist, his assistance and experience are 
necessary to confirm our results. M.Cauchoix very soon 
made several pairs of periscopic spectacles of different focal 
lengths for the purpose of trying their merits. For though 
Dr. Wollaston had given no measures for the different cur- 
vatures of the surfaces, M. Cauchoix, who is conversant 
with the theory as well as with the practice of his art, had 
ho difficulty in discovering such combinations of curvature 
as would answer his purpose. In those which he made 
first, the surface most curved was nearly concentric with 
the eye. The pupil might then be turned to the full extent 
of the glass on each side, and see (nearly) as well as through 
the centre. The field of view gained by this construction 
is really surprising, and it would require a person to be for 
Some time trained to the use of the common defective 
glasses, to be fully sensible of all the superiority of these. 
For my own part, I have not been accustomed to wear 
spectacles commonly, and have only used them occasionally 
for seeing distant objects; but for the last three months [I 
have regularly used the periscopic glasses, and I certainly 
never shall employ any others. 
There was, indeed, one inconvenience in those first con= 
structed by M. Cauchoix, which would be felt by those 
who are in the habit of wearing spectacles constantly. In 
looking towards a candle, particularly in a theatre where 
there are many lights, there appeared a variety of reflected 
images beside the principal object viewed, which occasioned 
some confusion. This arose from a combination of re- 
flections between the two surfaces, which, in consequence 
of the degree of difference of their curvatures, occasioned a 
distinct image to be formed on the retina after two reflec: 
tions. M. Cauchoix has however bappily succeeded in ré- 
moying this inconyenience altogether, by making the inner 
oi" Bb3 surface 
