180 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 692 Model of Double Convex Lens 



and Rays Converging. (Fig. 

 692.) The focal distance, or dis- 

 tance of the focus from the sur- 

 face of the lens, depends both 

 upon the form of the lens and of 

 the refractive power of the sub- 

 stance of which it is made ; in a 

 glass lens, both sides of which 

 are equally convex, the focus is 



situated nearly at the centre of the sphere, of which the 

 surface of the lens forms a portion ; it is at the distance, 

 therefore, of the radius of the sphere. Price, $1.00. 



Fig. 693. 



in the model. 



Model of Piano- Convex Lens 

 and Rays Converging. (Fig. 

 693.) Lenses that have one side 

 flat and the other convex, are 

 called plano-convex, and their 

 focus is at the distance of the 

 diameter of a sphere, of which 

 the convex surface of the lens 

 forms a portion, as represented 

 Price, $1.00. 



Fig. 694. 



Models of the Eye. (Figs. 694, 695, 696, and 697.) 

 These consist of four instruments, and form a beautiful 

 arrangement for showing the structure of the eye, and the 



nature of vision ; they are usually sold 



together. 



Model of the Jye.(ig. 694.) This 

 consists of a globe four inches in diame- 

 ter, and when the various parts are put 

 together representing the eye ; in front 

 is represented the cornea, and nearly 

 on an opposite part the optic nerve. 

 This globe is formed of two hemispheres, 

 the one fitting within the other for a 

 small space, so that they may be sepa- 

 rated easily ; within are parts represent- 

 ing the different coats and humors of 

 fci, the eye, in a clear and distinct manner ; 



