LABORATORY MANUAL OF PHYSIOLOGY. 



Duration of Impressions. On a circular white disc, mid- 

 way between the periphery and centre, fix a small black ob- 

 long disc. Rotate it rapidly. Note that a ring of gray appears 

 on the black, showing that retinal impressions are of a certain 

 duration. 



Inversion of Shadows. Make three pinholes in a card, close 

 together and arranged in a small triangle. Hold the card about 

 12 to 15 cm. from the right eye. Look through the holes at a 

 bright light. Close the left eye and hold a pin in front of the right 

 eye, so that it just touches the lashes. Note that an inverted 

 image of the pin will be seen in each hole. Retinal images are 

 inverted. Shadows are erect. Therefore the latter, upon being 

 projected outward into space, are seen inverted. 



NORMAL VISION. 



Examination of Distant Vision. The sense of sight consists of 

 (i) form sense (acuity), (2) light sense, (3) color sense. 



The acuteness of direct vision is measured by means of letters, 

 sized to certain definite standards. Those devised by Snellen are 

 in most common use. Snellen determined the normal acuteness 

 of vision to be the power of distinguishing letters subtending the 

 visual angle of 5'. The letters are formed of strokes whose width 

 is one-fifth the size of each letter, hence they are 'seen under an 

 angle of i'. The openings in the letters, and the spaces between 

 the contiguous strokes, are made to conform, as nearly as possible, 

 to the same angle. 



The relation of the size of the letter to the distance at which it 

 should be discerned by the normal eye is expressed by twice the 

 tangent of half the angle of 5', or, 0.001425. The size of the letter, 

 the perception of which constitutes normal vision at a given dis- 

 tance, may be obtained by multiplying the distance by 0.001425. 

 On this, the standard letters of measuring visual acuity have been 

 built up. 



Practical experience, however, has shown that letters constructed 

 under the angle of 5' do not always give the best visual acuity of 



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