Microscope Construction, Use, and Care 191 



slightest lateral motion of the head cuts off part ol the field, and 

 prolonged use produces marked fatigue. 



AV^ide-field oculars (noncompensating) have an exceptionally 

 wide field and good correction for curvature but may have a restricted 

 eye position as in the Hat-field type. This objection may be raised 

 concerning high-eyepoint oculars, which are designed to permit the 

 use of spectacles by the observer. 



Workers who must use spectacles with low-eyepoint oculars find 

 that the lenses of the spectacles and oculars become scratched after 

 some use. A simple remedy is to paste a narrow ring of felt over each 

 ocular. This permits the user to press his glasses against the ocular 

 and to utilize the full field, without damage to the glasses or the 

 ocular even after years of use. 



Illumination 



The most common method of ilhuiiinaiing a slide or other 

 transparent object is by transmitted light. 1 he light is projected 

 through the hole in the stage and passes through the preparation. The 

 simplest device for projecting light through the specimen is a concave 

 mirror under the stage, designed to focus a converging cone of ravs 

 at the level of the specimen. Regardless of the character of the light 

 source, whether daylight or artificial light, the curried mirror should 

 be used if the microscope has no condensing lenses under the stage. 

 The intensity of the illumination is controlled either by an iris 

 diaphragm, or by a rotating disk ha\'ing a series of holes of different 

 sizes. 



Microscopes that are used for advanced work are usually ecj[uipped 

 with a condenser. A condenser is a system of two or more lenses under 

 the stage, designed to receive a beam of parallel rays from a flat 

 mirror or a prism and to converge the light at the level of the stage. 



The simplest type of condenser, known as the Abbe condenser, 

 consists of two lenses. Although Abbe condensers are not corrected 

 for color or curvature, they are adequate for classwork and for much 

 of the routine work in research. The N. A. is 1.20 or 1.25. The upper 

 lens may be unscrewed (not in an elementary laboratory!) ; the lower 

 lens then serves as a long focus condenser of N. A. 0.30, suitable for 

 use with objectives of lOX (N.A. 0.25) or less. On some Leitz models 

 the upper element of the condenser is on a swinging yoke, whereby the 

 upper lens can be swung aside, leaving the lower lens as a long focus 

 condenser that fills the field of the lowest powers. A three-lens con- 

 denser with N.A. 1.40 is available for use with objectives having an 



