IMAGE 89 



Objectives and Eyepieces. The most important part of the modern 

 compound microscope is the objective and next to it the eyepiece. 

 Upon these, particularly the former, depends the clearness of definition 

 and details of the image obtained. 



Refraction. The natural law upon which the whole construction 

 of the optical parts of the microscope (objectives and eyepieces) 

 depends is that rays of light are deviated from their course when they 

 travel from a transparent medium of a certain density into a trans- 

 parent medium of a different density. This deviation from their 

 path, which occurs according to very definite mathematical rules, 

 is called refraction. Objectives and eyepieces can be so arranged 

 that the light after being refracted forms an enlarged or magnified 

 image in the eye of the observer. . Objectives are of low, of medium, 

 and of high magnification. 



Aberration. High-power objectives or lenses must be corrected 

 to overcome two sources of defects which interfere with the clearness 

 of the image. White light is composed of a number of component 

 colors (the colors of the spectrum or rainbow) and the rays of dif- 

 ferent colors are refracted in a different manner by refractive media. 

 Hence, they furnish an indistinct picture, not true in color, and which 

 is confused by the appearance of color rings. This defect of high- 

 power lenses is called their chromatic aberration. The other defect 

 is due to the very strong curvature and very small radius. A lens so 

 constructed will refract the rays at its periphery (margin) differently 

 from the other rays, forming indistinct pictures. This defect is called 

 its spherical aberration. Both the spherical and the chromatic 

 aberration can be corrected by a combination of lenses made up of 

 glasses with a difference in their refractive index. But the more com- 

 plete the correction the more difficult and delicate is the construc- 

 tion of these lenses, and, therefore, the greater the cost. 



Focus. Every microscopic objective is a system of lenses which 

 acts as a convex lens, and parallel rays of light passing through it are 

 refracted so that they meet in a single point called its main or prin- 

 cipal focus. The distance of this point or focus from the central 

 point of the lens is called its focal distance. In microscopic objec- 

 tives the higher the magnification the shorter will be the focal dis- 

 tance, and the lower the magnification the longer this distance. 



Image. If an illuminated object is placed somewhere between 

 the single and the double focal distance of a convex lens there is 

 formed on the other side of it an enlarged or magnified real image 

 of the object. In the use of the compound microscope the object 

 to be looked at is placed within the proper focal distance from the 

 objective and a real magnified image is then formed in the interior 

 of the draw-tube. The eye sees this through the ocular or eye- 

 piece, which again enlarges the real image in the draw-tube, and 

 forms in the eye a highly magnified visual image. This is the optical 

 principle of the construction and use of the microscope. 



