4 The Microscope 



The objective at the front end of the barrel is a powerful "magnifying 

 glass" that produces an enlarged image of the object O at 7 X . The extent 

 to which this image is magnified is called the "primary magnification," 

 and nowadays this figure is usually engraved on the barrel of the ob- 

 jective. The lowest power on laboratory microscopes is usually X3.5 or 

 X4. The highest is either X45 or, if there is an oil immersion, X90. 



This magnified image 7i is examined through a telescope, known as the 

 ocular or eyepiece, at the top end of the barrel. The first lens of the 

 ocular produces a magnified image of 7i at 7 2 , and the second lens of the 

 ocular produces a small image— usually about a millimeter in diameter— 

 of 7o at 7 3 . This is a real image that may be demonstrated by holding a 

 piece of translucent paper just above the eyepiece and lowering it until 

 the small disc-like image is sharply defined. This image is called 

 "Ramsden's disc." 



Ramsden's disc is rather small and it requires the human eye to trans- 

 form it into the illusion of a magnified image of the object. This is 

 done by advancing the eye until Ramsden's disc (7 3 ) is just inside the 

 cornea. Under these circumstances the lens of the eye casts an image of 

 Ramsden's disc over the whole surface of the retina at 7 4 . The image, in 

 fact, "fills the eye." The extent of the apparent magnified image can be 

 obtained by extending the dotted lines running from the lens to the 

 retina as far as the plane of the object O. The production of a real magni- 

 fied image on a photographic plate will be discussed later. 



The distance between the outer surface of the top lens of the ocular 

 and Ramsden's disc (7 3 ) is known as the "eye relief" of the ocular. 

 In most oculars it is so stupidly short that wearers of spectacles have 

 to remove them in order to get Ramsden's disc into the cornea and thus 

 fill their eye with the image. A few manufacturers offer a limited range 

 of oculars with "long eye relief" and it is ardently to be hoped that these 

 will in time become universal. 



Aberrations of Lenses. The distortions of shape and color in the images 

 made by simple lenses tormented the early makers of microscopes. These 

 distortions are due to two simple facts illustrated in Fig. 2, a section of a 

 glass prism with a beam of light going through it. The beam, entering 

 from the left, is of so-called "white" light. That is, it is a pencil of rays 

 of mixed wavelengths which, when combined on the retina, cause the 

 sensation that we have learned to call white. When a ray of light 

 passes from a medium of one optical density to a medium of another— as 

 from air to glass, or glass to water, or water to air— it is bent or "re- 

 fracted." The extent to which it is bent depends first on the angle at 

 which it enters the new medium and second on the difference in optical 

 density between the two media. This difference is usually expressed as 

 an index of refraction, which is the relative optical density of the 



