ELEMENTARY TECHNOLOGY. 



Indices of Refraction. 

 From vacuum to air 1.00294 

 " water 1.336 

 " Canada balsam 1.540 

 " Crown glass 1.530 

 " Flint glass 1.6 

 " glycerine 1.475 

 " oil of cloves, etc., 1.535 

 " diamond 2.439 

 j. Visual angle* 



The angle formed by the intersection of the rays from the 

 extreme points of an object. All objects with the same visual 

 angle appear of the same size. Thus, i, 2, 3, Fig. 3, PL i, 

 although different, appear of the same size because of the 

 same visual angle. 



We bring a minute object near the eye to increase the visual 

 angle and get as large an image as possible. The structure of 

 the eye makes a limit to this variety yet average standard 

 of distinct vision 10 inches. A convex lens increases the 

 power of the crystalline lens of the eye and enables an object 

 4n its focus to be seen at a larger visual angle, or magnified. 

 Comparing focal lengths of lenses with the average standard (10 

 inch) a lens of I inch focus magnifies 10 diameters j= 20, 

 # = 40, etc. 

 4. Faults of lenses. 



I. Spherical aberration. Errors due to the spherical surface 

 of a lens. 



The axial ray A, Fig. 4, PI. I , being perpendicular to the 

 surface of the lens, is not refracted. Rays near the axis, as C 

 C, are slightly bent and focused at c, but lateral rays, as R R, 

 are bent to r, so that the margin of the image formed at c is 

 misty and illy defined. Spherical aberration may be somewhat 

 corrected by intercepting the lateral rays with a stop or a dia- 

 phragm, but there is loss of light. 



Dr. Woolaston showed that spherical aberration could be 

 nearly removed by combining two plano-convex lenses of focal 

 lengths as i to 3 with plane sides to object, this more im- 



