t 
520 
PERISCOPIC SPECTACLES. 
common defective glasses, to be fully sensible of all the supe- 
riority of these. For ray own part, I have not been accustomed 
to wear spectacles commonly, and have only used them occa- 
sionally for seeing distant objects j but for the last three mouths' 
I have regularly used the periscopic glasses, and I now shall 
never employ any others. 
Tiiere was, however, one inconvenience iu those first cort- 
structed by M. Cauchoix, w'hich would be felt by those who 
are in the habit of wearing spectacles constantly. In looking' 
towards a candle, particularly in a theatre W’here there are 
many lights, there appeared a variety of reflected images, be* 
side the principal object viewed, which occasioned some con* 
fusion. This arose from a combination of reflections between ’ 
the surfaces, which, in consequence of the degree of difference ’ 
of their curvatures, occasioned a distinct image to be formed^ 
on the retina after two reflections. M. Cauchoix has, how*’ 
ever, happily succeeded in removing this inconvenience alto- 
gether j by making the inner surface ' of the glasses less con- 
cave^ than'he'did at first, so that whatever light may enter the '- 
eye after reflection ishio longer brought to a focus, and'*cori- 
sequenlly is not perceived. We have then a larger field thati 
with common spectacles, without introducing any new incon* 
veiiience. 
During the last three months M. Cauchoix has made trial* 
of- these spectacles on a great number of persons, and even ' 
upon one so short-sighted, that he could not see beyond the 
distance of 2^ inches, which is certainly a case of extreme ■ 
ihtortstghtedness. AH these persons agree in making the sarafe^ 
favourable report. The trials made by elderly persons requiring^ 
tire assistance of convex glasses, have also been attended w'itb' 
just the same success. 
I am the' more particular in noticing these trials of some' 
months’ contimJance, because it is by continued trial alone that 
we can be certain of the goodness of spectacles, and in general 
of- optical instruments that do not magnify much. The eye’ 
• As they haVe been' made fVolii the first by Messrs. DoHond. 
ha» 
