Ch. I] VISUAL ANGLE 9 



glass helping the eye (fig. 5-6). In the projection microscope there 

 is an actual or real enlarged image on a screen which the observer 

 looks at as if it were a large object (fig. 1). If one keeps in mind that 

 virtual images are purely imaginary, and that real images are pro- 

 duced by actual rays of light, it w r ill help to avoid confusion and 

 wrong interpretations. In every case where an object is seen, light 

 rays must pass from the object to the eye, and these rays entering 

 the eye must form an image on the retina. It is the retinal image 

 which furnishes the brain the stimulus for vision. 



Apparent Size of Objects 



Whether one is using a microscope or not, the apparent size of any 

 object seen depends upon the visual angle. Practically the entire 

 purpose subserved by the microscope is that it enables the eye to see 

 objects under a greater visual angle than would be possible without 

 the artificial aid. 



§ 6. Visual angle. — This is the angle made by the border rays 

 of light from the object to the retina, and crossing at the nodal point 

 or optical center of the eye (fig. 75—76). 



As the visual angle depends upon the distance the object is sepa- 

 rated from the eye, any means by which the object can be brought 

 closer to the eye will result in giving a larger apparent size to the 

 object, or in magnifying it. The lenses of the microscope used with 

 the eye enable it to get very close to the object and thus increase the 

 visual angle, and depending on the closeness, finer and finer details 

 of the object are separated, for they subtend an angle of one minute 

 or more (see § 226), and the object as a whole has a much greater 

 apparent size. (For further discussion see Ch. V.) 



§ 7. Pin-hole card. — Use a piece of paper about the size of a 

 library card. If the slip is black or of a dark color it makes the experi- 

 ment a little easier than when white paper is used. Make a hole in 

 this with a needle (fig. 7). If now one holds the slip up close to the 

 eye and gets the hole in the optic axis, the eye can see brilliantly 

 lighted objects very clearly. If, to start with, the object is off about 

 1 meter, quite an extent of it can be seen, and it will appear small. 

 Now go up closer and closer, and still the object is clearly seen, and 



