Microscope Construction, Use, and Care 199 



to 12 X- Magnifications up to 20 X are available, but, as the 

 magnification increases, the size of the field and the working distance 

 decrease. 



A compound microscope is one in which a lens unit, the objective, 

 produces a magnified image, which is in turn magnified by a second 

 lens unit, the ocular. By far the most common type of compound 

 microscope employs one objective and one ocular in working position 

 at one time. This is known as a monocular monobjective microscope. 

 This type is durable, has a wide range of usefulness, and permits full 

 use of the performance capacities of the optical system. The principal 

 objection is that the user employs one eye at a time, and the tendency 

 to use one eye more than the other causes excessive eyestrain and 

 fatigue, 



A binocular monobjective microscope uses a matched pair of 

 oculars with a single objective. A system of prisms in the binocular 

 body tube splits the beam coming from the objective and produces 

 two images of identical magnification and intensity. The use of both 

 eyes diminishes eyestrain and fatigue, and there is an impression of 

 depth and perspective to the visual image. Ocular tubes of the 

 binocular body are parallel in the majority of the principal makes. The 

 tubes converge at an angle in the standard Spencer binocular, but this 

 firm will furnish parallel tubes. Convergent tubes present the image to 

 the eye as if the image were at ordinary reading distance. When using 

 parallel tubes the eyes are relaxed, as in looking at an object at a 

 considerable distance. Some microscopists are convinced that they can 

 use only one or the other of these two types of binocular with comfort, 

 whereas other workers can use either type effectively. The binocular 

 body has adjustments for separating the ocular tubes for the inter- 

 pupillary distance of the observer. One ocular tube has a vertical 

 adjustment for correcting slight differences of focus of the two eyes. 

 To make this adjustment, select a minute structure in the specimen, 

 close the eye over the adjustable tube and focus on the object with the 

 fixed tube. Now close the eye over the fixed tube and bring the image 

 into sharp focus in the adjustable tube with the focusing device on this 

 tube. 



The quality of the image obtained with binocular bodies is equal 

 to that obtained with the single tube. Supplementary binocular bodies 

 that are designed to be placed upon older monocular microscopes, 

 have the tube length increased by the superimposed binocular body. 

 A reducing lens system must therefore be used to bring the magnifica- 

 tion back to the standard designated value. The most modern, and 



