CH. I] OCULARS FOR THE MICROSCOPE 23 



binations. By means of a screw collar the combinations may be 

 separated or brought closer together. If they are separated the 

 power is diminished; and if brought closer together the power is 

 increased. 



33. Illuminating or vertical illuminating objectives. These 

 are designed for the study of opaque objects with good reflecting 

 surfaces, like the rulings on metal bars and broken or polished and 

 etched surfaces of metals employed in micro-metallography. The 

 light enters the side of the tube or objective and is reflected vertically 

 downward through the objective and thereby is concentrated upon 

 the object. The object reflects part of the light back into the micro- 

 scope, thus enabling one to see a clear image. 



34. Low and high objectives. A low objective is one that mag- 

 nifies relatively little, and a high objective is one that magnifies the 

 real image greatly (fig. 21 B,C). By looking at the equivalent focus 

 of an objective one can, of course, tell very precisely concerning its 

 magnification ( 19, ipa), but it is also very convenient to judge some- 

 thing of the power by the general looks. As a rough statement it 

 may be said that a high power usually appears more elaborate than 

 a low power. The front lens is usually smaller, and the whole mount- 

 ing is usually longer. Conversely, low objectives are usually shorter 

 and the front lens larger than with high powers. 



OCULARS AND THEIR DESIGNATION 



35. An ocular or eye-piece for the microscope consists of one 

 or more converging lenses or lens systems next the eye. Its main 

 purpose is to act with the eye as a magnifier of the real image formed 

 by the objective (fig. 20). Incidentally the ocular also serves to cor- 

 rect some of the defects of the objective (see Ch. IX). 



Oculars may be divided into groups according to their construction 

 or action. 



36. Positive oculars. These have the real image of the objec- 

 tive formed below all the lenses of the ocular (fig. 22 A,B). 



37. Negative oculars. In these the real image formed by the 

 objective is between the lenses (fig. 23, 24). 



In a negative ocular the lower or field lens acts with the objective 



