DISTINCTNESS OF IMAGE. 



19 



crown glass that the dispersion of one is corrected or compen- 

 sated by that of the other, while the two still act like a double 

 convex lens in magnifying the object (Fig. 19). Chromatic 

 aberration cannot be corrected absolutely; there will always be 

 a little color, but it may be so little that the image is practically 

 colorless. For optical purposes the blue and orange are cor- 

 rected or combined. If the image is bordered by a light blue 

 fringe, the lens is said to be overcorrected ; if by a reddish one 

 it is undercorrected. 



A lens free from chromatic aberration is called Achromatic 

 and one free from both spherical and chromatic aberrations ia 

 called Aplanatic. 



Fig. 18. 



Fig. 19. 

 a crown lens, 

 b flint lens. 



SIMPLE MICROSCOPE. This is nothing but a convex lens used 

 as a magnifier as described under double convex lens. There 

 may be one lens or several combined into a system and mounted 

 in a suitable stand. Corrected lenses and diaphragms may be 

 used to get rid of spherical and chromatic aberrations. A good 

 example of a simple microscope is a reading glass or a watch- 

 maker's magnifier. 



CONDITION OF DISTINCTNESS OF THE IMAGE. There is for each 

 person a distance of most distinct vision, a distance at which 

 an object must be placed before the eye to be seen with greatest 

 distinctness. This distance is, for the average eye, between 12 

 and 14 inches. It differs for different observers, and the two 

 extremes are found in near and far-sighted persons. When an 

 object is looked at through a lens the latter must be moved 

 back and forth until the image is formed at the particular 

 observer's distance of distinct vision, and this operation is called 

 focusing. This explains why two persons looking through a 

 microscope will have quite different foci, since the two eyes 

 have different distances of distinct vision. 



The human eye is, in one respect, an optical instrument, be- 

 cause it consists of a combination of lenses which focus upon 

 the retina light rays coming from illuminated objects. The 

 eye has but little spherical aberration, owing to its peculiar 

 shape and to the action of the iris, which takes the place of a 

 diaphragm ; but it does have considerable chromatic aberration. 

 Most eyes have power of accommodation, that is, of altering 



