vi DIOPTEIC MECHANISM OF THE EYE 327 



the whole surface of the retina. The arteries are easily recognised 

 by their brighter red colour, and by the stronger reflection of light 

 from their surface. The hue of the funclus may vary from light- 

 red to brownish-red, according to the amount of pigment it con- 

 tains. Within physiological limits the appearance of the fundus 

 varies considerably in different individuals. 



Many attempts have been made to obtain a direct photograph 

 of the fundus of the eye. Among the more recent are those of 

 Fick and Gerloff (1891), Thorner (1896), Borghi and Bonacini 

 (1898), Nicolaew and Dogiel (1900-1903), Bajardi (1905). But 

 Dimmer (Graz, 1899-1904) alone succeeded in obtaining almost 

 perfect photographs of the fundus by exposures of yV-V sec. 

 (Fig. 152). 



The most important part of the retina visible by the ophthal- 

 moscope is the so-called yellon' 

 spot (macula lutea), which is dis- 

 tinguished by its lighter colour 

 and absence of any visible blood- 

 vessels. The yellow tint that 

 gives rise to the name " macula 

 lutea " is a post-mortem pheno- 

 menon which is not present in 

 the freshly excised eye (Gull- 

 strand). 



The elliptical yellow spot has 

 at its centre a slight depression 

 known as the fovea centralis. 

 In the excised eye the diameter 



Of the yellow Spot is about 1-2 Fi. 102. Photograph of fundus of normal .-y.- 



mm.; the fovea only from 0-2- ft^T.'.r' 



0'4 mm. It is at this point that 



the image of the object which we focus is formed ; in other words, 



the visual axis passes through the fovea. In fact, for the observer 



to see the yellow spot and fovea with the ophthalmoscope, it is 



only necessary that the observed eye shall be turned directly on 



to his. 



Ophthalmoscopy not only enables us to ascertain the state of 

 the fundus, but by it the static refraction of the eye can be more 

 or less accurately determined. 



We said that when the observer's eye is emmetropic he is 

 able without a correction lens to see plainly the vessels in the 

 eye focussed on a distant object ; the eye investigated being also 

 emmetropic. If it is necessary to add a correction lens to the 

 ophthalmoscope, the observed eye is ametropic ; if a concave lens 

 is needed, it is myopic ; if a convex lens, hypemietropic. 



The degree of myopia and hypermetropia may be determined 

 from the strength of the weakest lens required to obtain a clear 



