MICROSCOPE AND ACCESSORIES 



[CH. I 



combinations of lenses, the one next the object beirig called the froni combina- 

 tion or lens, the one farthest from the object aBd nearest the ocular, the back 

 combination or system. There may be also one or more intermediate sys- 

 tems. Each combination is, in 

 general, composed of a convex and 

 a concave lens. The combined ac- 

 tion of the system serves to pro- 

 duce an image free from color and 

 from spherical distortion. In the 

 ordinary achromatic objectives of 

 the older period the convex lenses 

 are of crown and the concave 

 lenses of flint glass. In the best 

 modern achromatic objectives the 

 new Jena glass is used for a 

 part or all of the lenses. (Figs. 

 27, 28.) 



Fig. 26. Diagram showing 

 the principle of a compound micro- 

 scope with the course of the rays 

 from the object (AB) through the 

 objective to the real image ( B'A') , 

 thence through the ocular and into 

 the eye to the retinal image (A 2 !?'*), 

 and the projection of the retinal 

 image into the field of vision as 

 the virtual image (B^A 3 ) . 

 s A B. The object. A*B*. The 

 retinal image of the inverted real 

 image, (B'A 1 ), formed by the ob- 

 jective. B*As. The inverted vir- 

 tual image, a projection of the 

 retinal image. 



Axis. The principal optic 

 axis of the microscope and of the 

 eye. 'a 



Cr. Cornea of the eye. L. 

 Crystalline lens of the eye. R. 

 Single, ideal, refracting surface 

 at which all the refractions of 

 the eye may be assumed to take 

 place. 



F. F. The principal focus of 

 the positive ocular and of the ob- 

 jective. 



C, 



