Microscope Construction, Use, and Care 185 



inverted, and the direction of motion of the object is reversed. I'lic 

 apparent size of the virtual image is the same as if the observer viewed 

 the projected image on a screen 10 in. from the ocular. As a specific 

 case of magnification, let us view an object 0.1 mm. long with a 

 lOOX microscope. This produces a retinal image of the same size 

 and the same impression of magnitude as if we looked at the lO-mm. 

 image projected by the same microscope on a screen 254 mm. from 

 the eyepiece. 



Properties of Objectives 



MAGNIFICATION 



The most obvious property of objectives is magnification, which 

 is a fixed value under the conditions outlined in a preceding para- 

 graph. The objective magnifications used most commonly on standard 

 monobjective microscopes range from 3.2x to lOOX- Magnifications 

 below this range are used on paired-objective stereoscopic prism bi- 

 nocular dissecting microscopes. Objectives above lOOx have rather 

 limited uses. The conventional low-power objective is lOX- The 

 lower powers, from 3.2X to 6X. are not fully appreciated and deserve 

 more serious consideration. 



WORKING DISTANCE 



Free working distance is the distance between the objective and 

 the cover glass, using a cover glass 0.18 mm. in thickness. The cata- 

 logues of the manufacturers give the working distances of their 

 objecti\ es. A few selected illustrations show the relation between mag- 

 nification and working distance: lOx, 7.0 mm.; 43x» 0-6 mm.; 45x> 

 0.3 mm.; 95 X> 0-13 mm. It is obvious that for an elementary class 

 the most desirable high-power objective has a magnification in the 

 forties and the longest available working distance. Objectives of high 

 magnification and short working distance must be used with care 

 to avoid damaging the front lens and the slides. 



FOCAL LENGTH 



If a beam of parallel ra)s is projected through a simple lens, the 

 rays converge at a point. The distance from this point to the optical 

 center of the lens is the focal length. In an objective consisting of 

 several components, the situation is somewhat more complex, and 

 a different \alue is used. The manufacttners engrave on the mount- 

 ings and list in their catalogues a value known as the ecjuivalent 

 focus. At the standard projection distance of 250 mm., an objective 



