PRACTICAL EXERCISES 



1067 



of it, minus one diopter, the refraction can be estimated. Suppose, 

 for instance, that a convex lens of two diopters is required, then hyper- 

 metropia of one diopter exists. 



In order to facilitate the introduction of the various lenses, instru- 



Fig. 459 Geneva Retinoscope and Ophthalmoscope. A, frame of instrument: 

 B, retinoscope attachment; C, ophthalmoscope attachment; D, base; i, mirror 

 handle; 2, clip to hold the proper lens to correct the abnormality of refraction 

 of observer or patient when viewing the retina with the ophthalmoscope; 3, scale 

 indicating the meridian of handle and pointer; 4, ring in whick mirror cup rotates; 

 6, mirror; 7, mirror spring for reflecting the light to a given point; 8, screws for 

 adjusting mirror; 9, screw for holding light and ring 4 in position; 10, handle 

 for swinging A from side to side; 13, opening in iris diaphragm, controlled by 

 handle 14; 15, lamp hood; 17, knurled handle for rotating disc containing the 

 full diopter lenses; 18, handle for rotating the disc containing the fractional 

 lenses (white numbers indicate plus lenses, and red minus lenses) ; 20, opening 

 through which observer looks when adjusting the retinoscope to the patient's 

 eye; 21, pinion for advancing or retracting instrument; 24, bracket ring of 

 retinoscope attachment B, which is slipped over ring 25 when putting retinoscope 

 attachment into place; 28, clips for ' fogging ' lenses through which the patient 

 looks to relax accommodation; 29, opening through which the pupil is viewed in 

 retinoscopy; 30, opening containing clip in which extra lenses may be inssrted 

 when required, or the defect is over 8 diopters; 32, patient's eye-cup; 33, ring of 

 ophthalmoscope attachment C, which telescopes over 25; 34, ophthalmoscope 

 tube; 35, binding-screw which holds the instrument in a fixed position when 

 retinoscope is being used ; 37, rack to raise and lower the instrument; 40, handle 

 controlling height of chin-rest 44; 46, forehead -rest. 



ments called skiascopes or retinoscopes may be used, one of which is 

 shown in Fig. 459. 



14. Pupillo-dilator and Constrictor Fibres. (a) Set up an induction 

 machine arranged for tetanus, and connect a pair of electrodes through 



