LABORATORY MANUAL OF PHYSIOLOGY. 



is taken as the unit of measure. It is numbered i, and is called 

 one diopter (D). The refractive power of a lens is the inverse of 

 its focal distance. Hence a lens of 2 diopters has a focal distance 

 of 0.50. What is the focal distance of a lens of 4 D? Given a 

 lens whose focal distance is 2 metres, what is its number ? 



Parallel rays of light, passing through a convex lens, are made 

 to converge. Parallel rays of light, passing through a concave 

 lens, are made to diverge. 



A cylindric lens is a lens, one or both surfaces of which are seg- 

 ments of a cylinder. Rays of light, passing through it in a plane 

 parallel to its axis, are not bent. Rays passing in a plane perpen- 

 dicular to its axis, converge or diverge, according as to whether the 

 cylinder is concave or convex. 



Lenses are designated plus (+) if they are convex, and minus 

 ( ) if concave. 



What forms of ametropia would cylinders correct ? 



OPHTHALMOSCOPY. 



The ophthalmoscope is, in its simplest form, a mirror with a hole 

 in it. The first instrument of Helmholtz, in fact, consisted of 

 three thin plates of glass, fastened together and mounted in a frame, 

 at an angle of 56. The whole object of the instrument is to il- 

 luminate the ocular fundus by reflected rays of light and permit 

 the observer to inspect the illuminated area. All patterns are 

 useful. The patterns of Loring and Morton are most popular. 



Use. Not all the light entering the pupils is absorbed by the 

 pigmentary layer of the choroid. A certain amount returns from 

 the eye. If, therefore, the observer's eye is placed in the same 

 position as the source of illumination, or directly behind it, the 

 interior of the eye becomes visible. This is the principle of the 

 ophthalmoscope. The mirror, which gathers rays of light from a 

 luminous point, becomes a secondary source of light which is pro- 

 jected into the pupil. 



Methods. There are two, direct and indirect. In the direct, 

 the examiner places his eye close to that of the patient and looks 



[192] 



