PRACTICAL EXERCISES 



1063 



7. Spherical Aberration. Close one eye, and bring a small object 

 (a pin or the point of a pencil) towards the other eye till it becomes 

 blurred. Interpose between the object and the eye a card perforated 

 by a small hole. The object becomes more distinct owing to the 

 cutting off of the peripheral rays (p. 987). 



8. Chromatic Aberration. Look at Fig. 406 (p. 988) from a distance 

 too small for perfect accommodation, and verify the facts given in 

 the description of the figure. 



Fig. 454. Vertical Section of Ophthalmometer. d, outer tube of the telescope 

 rotating in sleeve or collar s (supported by siamlard t, which is swivelled in 

 tubular support, g) ; k, diaphragm ; 10, eye-piece with lenses a and b ; n, a station- 

 ary disc, borne on collar s, graduated to indicate angle of rotation of u, a black 

 concave disc rotating with tube d, and having fixed in it two illuminated figures 

 (or mires), w, w, whose images reflected from the cornea are observed ; MS a pointer 

 carried on the tube d which shows on the graduated arc the amount of rotation; 

 12, 12, hemispherical shells containing small incandescent lamps for illuminating 

 the translucent mires. The lamps are connected with wires running in the 

 hollow stem J; /is the inner tube of the telescope carrying the double prism, h, h. 

 By means of the rack o, projecting through the slot m, and engaged by the pinion 

 p, f is moved back and forth in the outer tube, thus approximating or separating 

 the corneal images of the mires. On the axis of p is a milled head for turning it, 

 and two duplicate discs graduated with a scale showing the radii of curvature 

 of the cornea in millimetres, and another scale showing their equivalent in 

 diopters. 



9. Measurement of the Extent of the Field of Vision. Use the peri- 

 meter shown in Fig. 430 (p. 1018). 



(i) For White Light. Fix in the holder, Ob, on the graduated arc, 

 a small piece of white paper, and put one of the charts supplied with 

 the instrument at the back of the wheel which revolves with the arc. 

 The observations can be recorded on this chart. The patient rests his 

 chin on K and adjusts one eye against O. This eye is kept fixed on 

 the mark at /during the whole period of observation, and the other eye 



