PERISCOPIC SPECTACLES. 



16. The most perfect vision with spectacles is produced when 

 the eye looks in the direction of the axis of the lenses, and more 

 or less imperfection always attends oblique vision through them. 

 Persons who use spectacles, therefore, generally turn the head, 

 when those whose sight does not require such aid merely turn the 

 eye. 



To diminish this inconvenience, the late Dr. "Wollaston sug- 

 gested the use of menisci, or concavo-convex lenses, instead of 

 double concave or double convex lenses with equal radii, which 

 up to that time had been invariably used. 



Sections of lenses of this kind are given in figs. 3 and 4. In fig. 

 3, the convexity A' B' c', of which the centre is o', is greater than 



Fig. 3. 



the concavity ABC, of which the centre is o, and the effect of the 

 lens is the same as that of a convex lens. Such glasses are there- 

 fore adapted for weak sight. In fig. 4, on the contrary, the con- 

 cavity A B c, of which the centre is o, is greater than the convexity 



Fig. 4. 



C' 



A' B' c', of which the centre is o', and the effect of the lens is the 

 same as that of a concave glass. Such glasses are therefore 

 adapted to short sight. 



The effect of these, as compared with double convex and double 

 concave glasses, is, that objects seen obliquely through them are 

 less distorted and, consequently, that there is a greater freedom 

 of vision by turning the eye without turning the head, from which 

 property they were named periscopic spectacles. 



17. In the selection and adaptation of spectacles, it is invariably 

 assumed without question, that the two eyes in the same indi- 



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