OPHTHALMOSCOPY OF THE VERTEBRATE EYE 



27 



the original ophthalmoscope of Babbage than 

 that of Helmholtz. 



It consists essentially of a concave mirror 

 of about 25 cm. focus (mounted on a handle), 

 with a small (3 mm.) hole in the center and 

 so arranged that a series of concave and con- 

 vex lenses may be presented to the opening 

 for the purpose of neutralizing the refractive 

 error of the observer's and the observed eye, 

 thus giving a clear view of the fundus details. 



There are numerous ophthalmoscopic mod- 

 els in the market today — improvements on 

 the early inventions — but the instruments 

 of Loring and Morton are, perhaps, the favor- 

 ite forms now in use. Illustrations of these 

 are given in the text. See Figs. 15 and 20. 



As the purpose of an ophthalmoscopic ex- 

 amination is to see as much as possible of 

 the interior of the eye under observation it 

 is desirable to dilate the pupil; — in the case 

 of mammals this is accomplished by instilling 

 into the eye atropin, homatropin, euphthal- 

 min or some other mydriatic. 



Befoe the invention of the self-luminous 

 or electric ophthalmoscope (Fig. 21) the source 

 of illumination of the ocular interior was a 

 gas jet or electric bulb — preferably an argand 

 lamp — stationed in a darkened room, and 

 placed on a level with the eye to the side of 

 the head. 



In examinations of the human eye the 

 observer sits opposite the individual whose 

 ocular interior is to be explored, and placing 

 the mirror close to his own eye and about 

 40 or 50 cm. from the eye to be examined 

 reflects the light upon the latter, while he 

 looks at it through the small opening in the 

 ophthalmoscopic mirror. In very short- 

 sighted and very long-sighted eyes, but not 

 in normal ones, the vessels of the retina, the 

 entrance of the optic nerve, etc., can be more 

 or less distinctly seen. 



The details of the retina, choroid, etc. (or 

 fundus), can be seen in two different ways. 

 In the indirect method, as applied to Man and 

 most other vertebrates, the observer, seated 

 as shown in the cut (Fig. 16), holds a strong 

 (16 d.) convex lens about 10 cm. from the 

 eye under examination, and between it and 

 his own, when a clear real image of part of 



the fundus, inverted and magnified about four 

 diameters, appears in the red light of the 

 pupil. 



In the direct method (Fig. 17) the observing 

 eye must be placed as close to the observed 

 as the intervention of the mirror will allow, 

 when a virtual image of a still smaller part of 



Fig. 15 

 Loring Ophthalmoscope 



the fundus is seen, but erect and magnified 

 about fourteen diameters. 



Direct ophthalmoscopy, or the examination 

 by the upright or erect image, is illustrated 

 by means of Fig. 17. A side lamp (L) is 

 used, the rays from which are focused by the 

 perforated mirror of the ophthalmoscope (00) 

 and thrown into the observed eye through the 

 dilated pupil (P). The rays of this pencil 

 now cross about X and impinge on the retina 

 and choroid at OO and cover the ocular back- 

 ground from B to B. Thence a sufficient 

 number of efferent rays are reflected and re- 

 fracted back through the ocular media of 

 both the observing and observed eyes to the 

 percipient elements of the former, at L. 



Another illustration (Fig. 16) shows indirect 

 ophthalmoscopy, or, as it is sometimes called, 

 the examination by the inverted image. A 

 divergent light pencil proceeds from the 



