508 TRANSACTIONS OF SECTION A. 



shape of tlie tufts Ilelmholtz suggests is due to the shape of the fovea, which (in 

 agreement with Brewster, though he does not refer to him) he considers is the 

 seat and cause of the appearance. 



Prior to either Brewster or Helmholtz, Clerlt Maxwell, in a communication 

 to the British Association in 1856, had shown how, by an absorbent medium, any 

 observer could see the yellow spot of his own eye,' and he remarks : ' By using a 

 Nicol the brushes of Haidinger are well seen in connection with the yellow spot, 

 and the fact that the brushes being the spot analysed by polarised light becomes 

 evident.' This is the earliest suggestion of the true origin of the tufts. The 

 best coloured screen for seeing the yellow spot is that suggested by Mr. Whitraell — ■ 

 viz., a weak solution of the double oxalate of chromium and potassium, which 

 is quite opaque to the yellow and orange rays. Looking at the sky or any 

 illuminated surface through such a screen, everyone can distinctly see the yellow 

 spot in his eye, as a nearly circular dark red patch. If now a Nicol be held before 

 the eye, Haidinger's tufts are seen exactly coincident with the yellow spot, the 

 longer axis of the tufts corresponding to the diameter of the yellow spot. The 

 macula lutea and the tufts (or fascicula lutea) are therefore identical in position 

 in the retina, though their shape ditlers. The faint blue light seen by some but 

 not all persons at right angles to the yellow tufts can hardly be, as Jamin has sug- 

 gested, a contrast e£[ect — the blue tint being complementary to the yellow — for 

 the blue tint is seen when a polished black surface is looked at, and some persons 

 see the blue sectors better than the yellow. 



I am disposed to think that the production of the tufts is due to the combined 

 action of the bi-refracting film of the cornea, together with the polarising structure 

 of the yellow spot. That neither the cornea nor the crystalline lens are alone 

 concerned in their production has already been shown, and can be conclusively 

 demonstrated by using the instrument which I have termed the entoptiscope, a 

 description of which is published in recent ' Proceedings ' of the Jloyal Dublin 

 Society." A simple device can, however, be employed. An opaque screen with 

 two pin-hole apertures, each 1 mm. in diameter and 2 mm. apart, held between 

 the eye and the Nicol's prism, would give a double image of Haidinger's tufts if 

 they were due alone to the refracting media of the eye ; but only a single image 

 of the tufts is seen ; hence their detection is due to the retina. 



No observer has hitherto measured the actual retinal area occupied by 

 Haidinger's tufts. This can readily be found if the distance of the nodal point, 

 or optical centre of the eye, from the retina is known. In a normal eye the 

 nodal point is 10 mm. from the retina. Now the angle subtended by the tufts is 

 between 3° and 4°. Calling this angle n, and the distance of the nodal point k, 



the retinal image will be = 2k sin -, which gives a length of about 1 mm. for the 



longer axis of the tufts. A more accurate method is to measure the length of 

 the projected image of the tufts on a white sheet of paper at a known distance 

 from the eye. The distance of the nodal point from the retina being known (and 

 this distance varies from 13 mm. in hypermetropic eyes to 18 mm. in myopic 

 eyes), the retinal area of tbe tufts can be accurately found. In my own case, and 

 in that of two or three of my assistants who have tried, I find the retinal area 

 major axis of the tufts to be from 08 to I'O mm. 



Now the longer axis of the fovea centralis is only 0*2 mm. The tufts 

 cannot, therefore, be restricted to the fovea. But the diameter of the yellow spot, 

 the macula lutea, exactly corresponds to that of the tufts — viz., 0'8 to I'O mm. 

 Hence Haidinger's tufts are not due, as Brewster and Helmholtz thought, to the 

 fovea, but to the whole yellow spot, and doubtless .are caused by some polarising 

 property posse.ssed by the structure of the baciUary layer of the retina in that 

 spot. Throughout the yellow spot the rods of the baciilary layer are absent, the 



' The use of an absorbent medium, a weak solution of chromium chloride, for 

 observing the yellow spot in one's own eye was, as Clerk Maxwell points out, first 

 suggested by Stokes. 



'' Proc. Roy. Bub. Sec, May 1906. 



