OPTICAL INSTRUMENTS. 



151 



Periscopic Glasses. (Fig. Fi e- 626 - 



626.) Dr. Wollaston, in order 

 to allow the eyes a consider- 

 able latitude without fatigue, 

 invented a peculiar form of 

 glasses, called by him peri- 

 scopic, from two Greek words 

 signifying seeing about ; their 

 form is that of a meniscus with 

 the concave side always turned towards the eye. When 

 they are intended for long-sighted persons, or old age, the 

 anterior surface, or that next the object, is formed spheri- 

 cally convex, with a curve deeper than the concave, so as 

 both to gain the required power, and compensate for the 

 divergency occasioned by the concave side ; this form is 

 shown at A, in the above cut. The periscopic form em- 

 ployed for correcting the defect of a short or near-sight is 

 shown in section at B, having its anterior surface convex, as 

 in the former case ; but here the concavity on its posterior 

 side is increased to procure the required divergency, and 

 compensate for the convex side. 



Pebbles are made from blocks of rock crystal, usually 

 brought from the Brazils ; are cut into slices by the lapidary 

 and ground to convex, concave, or periscopic forms by the 

 optician for spectacles. They have the advantage of being 

 more transparent than glass, much harder, do not scratch, 

 take a higher polish, and consequently transmit more light ; 

 also are cooler to the eyes. The price of the best convex 

 pebbles is usually $2.25, and for concave $3.25, or where 

 they are purchased with the frames $1.75 extra, for convex, 

 and $2.50"for concave. 



