Relation betiveen Capillary Pressure and Secretion. 439 



5. The fatiguing light should chiefly affect the region used for the fatigue. 

 This is not the case. 



6. An after-image should not be seen in the absence of all external light, 

 except such as would arise from the Eigenlicht if this subsidiary hypothesis 

 of Helmholtz be allowed. In the latter case the after-image should change 

 colour as in all other cases according to this theory. 



7. On the Young-Helmholtz theory yellow should change to green after 

 fatigue to red, or to red after fatigue to green. This is not the case. 



The Relation betiveen Capillary Pressure and Secretion. II. — The 



Secretion of the Aqueous and the Intraocular Pressure. 

 By Leonard Hill, F.B.S., and Martin Flack (Eliza Ann Alston Eesearch 

 Scholar). 



(Eeceived May 29,— Eead June 27, 1912.) 



(From the Physiological Laboratory, London Hospital Medical College, London Hospital 

 Eesearch Fund.) 



Many observations have been recorded of the aqueous or intraocular 

 pressure. The method generally used has been to pass a hollow needle 

 through the cornea and connect this with a manometer. J. Herbert 

 Parsons* says : " The first essential is . . . that no fluid shall enter or leave the 

 eye whilst the needle is being inserted." It is no less essential that the 

 aqueous shall not escape along the track of the needle or into the mano- 

 meter. Although expressing the opinion cited above, Parsons gives the 

 figure of a needle employed by him, which not only has an open end, but 

 also two side holes, one nearer to, one farther from, the open end ; so 

 contrived, he says, to give free admission to the aqueous fluid. Such a 

 needle seems to us especially liable to allow the escape of the aqueous 

 during insertion. Starling and E. E. Hendersonf used a needle with an open 

 eyehole, from which fluid was allowed to escape at a pressure conjectured 

 to be greater than the pressure of the aqueous. By this means they 

 believed to prevent the escape of aqueous during insertion. It is not 

 certain that this means is effectual, since pressure on the eyeball raises the 



* 'The Pathology of the Eye,' vol. 3, p. 1049. Hodder and Stoughton, London 

 (undated). 



t ' Journ. Physiol.,' 1904, vol. 31, p. 305. 



