CH. IX] MICRO-PROJECTION FOR LARGE CLASSES 227 



The fresh blood corpuscles of man, for example, are about 7.5/4 

 in diameter. To see these as discs on a screen at a distance of 10 

 meters would require a magnification of 4,000 and preferably of 

 8,000 diameters. With such a high magnification the sharpness of 

 the outline, and the distinction between the corpuscles and the 

 medium in which they float is almost lost, and there is nothing but 

 a vague haze with shadowy outlines. If one goes up closer to the 

 screen to see the images well, one will be sorely disappointed, for 

 they are vague in outline and wholly unsatisfactory as compared 

 with the appearance gained by looking directly into a microscope 

 ( 355a). 



355a. Visibility of objects or their magnified images. It has been found 

 by careful observation and experiment that the most sensitive part of the eye 

 is in the fovea centralis or yellow spot; and that in order to see two points, by 

 the fovea, as separate, they must be far enough apart so that the visual angle 

 is one minute. If the visual angle is less than one minute, two points appear to 

 most eyes as one. 



The question now is, how far separated must the parts of an object be in 

 millimeters or inches in order that the form of the object can be distinguished. 

 To answer this it is necessary to know the actual length of the one minute of arc 

 when the eye is at different distances. 



To determine the length of one minute of arc in any case, the eye is con- 

 sidered to be at the center of a circle and the object at the circumference, and 

 no matter how great the visual distance, the object must subtend one minute 

 of the arc of the circle of which the visual distance is the radius in order to have 

 its parts distinguishable. 



To determine the actual length in millimeters or inches of one minute of arc 

 in any circle, it is only necessary to remember that the circumference of a circle 

 is 6.2832 times its radius and that it is divided into 360 degrees or 21,600 

 minutes (fig. 125). 



If, now, the radius of the circle, or the distance of the eye from the object is 

 I meter, the circumference of the circle will be 6.2832 meters or 6,283.2 milli- 

 meters. As there are 21,600 minutes in the circumference, the length of one 

 minute is 6,283.2 mm. --.- 21,600 = .2908 mm. or approximately .3 mm. That 

 is, with the eye at one meter distance, the parts of an object should be separated 

 .3 mm. to be seen as distinct points. 



For the standard distance of distinct vision (25 cm.), used in microscopic 

 magnification, the object must be Kth this size or .075 mm.; and for a dis- 

 tance of i o meters it must be 10 times as great or 3 millimeters, and for 6 meters, 

 the distance used for testing vision, it must be .3x6 = 1.8 mm. 



A greater separation of the points is desirable for the most accurate deter- 

 mination, but those given above are the minimum for most observers. 



Now to apply the above to the magnification necessary for a screen image of 

 the human blood corpuscle which has a size of 7.5," (.0075 millimeters; .000295 

 inch). To give the necessary sized screen image of .3 mm.; .075 mm. and 3 

 mm. at distances of i meter, ^th meter, and 10 meters, it is only necessary to 

 divide the size of the screen image in each case by the size of the object (7.5^ 

 or .0075 mm.). 



