PERISCOPIC SPECTACLES. 
319 
ince the pupil of the eye is of very small size, it is, in fact, 
>ut a very small portion of a spectacle glass that is employed 
n any one position of the eye, though its several parts may bo 
ised in succession when ajiy lateral motion is given to the eye. 
rle thence inferred, that the form usually given to such glasses, 
hough well adapted for other uses in which the rays from all 
»rts of the glass are to be collected into one focus, is cot the 
jest for spectacles ; but that the best construction would be 
hat which would give to all parts, separately, the same power 
>f assisting the sight when the eye is turned to each of them 
n succession. Dr. Wollaston was thus led to the obvious con- 
tusion, that the form should be {iomUe) convex without and 
.oucave within, so that rays coming to the eye would pass 
nearly at right angles to the surface of the glass in ail directions. 
These glasses were called by the inventor periscopic, and the 
jxclusive sale of them was secured to Messrs. Dollocd, by pa- 
isnt. 
My attention having been some time since drawn to this sub- 
ject by au article in Nicholson’s Journal, I proposed a trial of 
.hem to Al. Cauchoix, well known as a skilful optician in gene- 
lal, and more particularly by the large achromatic lenses which 
se has lately made of flint glass manufactured in France by 
11. Dartigues. 1 requested his opinion on the subject; for, 
uough our theory should direct the artist, his assistance and 
uperience are necessary to confirm our results. M. Cauchoix 
f?ry soon made several pair of periscopic spectacles of different 
:ocal lengths for the purpose of trying their merits. For, 
Uough Dr. Wollaston had given no measures for the different 
tarvatures of the surfaces, M. Cauchoix, who is conversant 
tilth the theory, as well as with the practice of his art, bad no 
-•fficulty in discovering such combinations of curvature as 
t ould answer his purpose. In those which he made first, 
k»e exterior surface was nearly concentric with the eye. The 
tapil might then be turned to any extent on each side, and see 
nearly) as w’ell as through the centre. The field of view 
uined by this construction is really surprising, and it would 
iLquire a person to be for some lime trained to the use of the 
commoB 
