358 Mr. W. Crookes on the Measurement of the 



in the brain or spinal cord, and has stated to occur in the retina and else- 

 where. 



In all other mammals, except the hedgehog, as far as my present know- 

 ledge extends, the arteries, veins, and capillaries lie in the retina. In fish, 

 amphibia, reptiles, and birds, however, as H. Miiller and others (myself 

 as regards amphibia and reptiles) have shown, the retina is absolutely non- 

 vascular, the absence of proper retinal blood-vessels being compensated for 

 in fish, amphibia, and some reptiles by the vascular net which in these 

 animals channels the hyaloidea, and by the highly vascular pecten present 

 in other reptiles and in birds. Thus it is possible to divide vertebrates into 

 two classes, according as their retina is vascular or non- vascular ; and these 

 classes would be connected by the hedgehog, the larger branches of whose 

 vasa centralia lying upon the membrana limitans in intimate relation with 

 the hyaloidea, represent the equivalent vessels of the hyaloid system, which 

 forms so exquisite a microscopic object in the frog ; whilst the capillary 

 vessels channelling the retinal tissues occupy the same position which they 

 do in most mammalia. 



[The drawings in illustration of this paper are preserved for reference in 

 the Archives of the Royal Society.] 



" On the Measurement of the Luminous Intensity of Light." By 

 William Crookes, F.R.S. &c. Received June 27, 1868*. 



The measurement of the intensity of a ray of light is a problem the solu- 

 tion of which has been repeatedly attempted, but with less satisfactory 

 results than the endeavours to measure the other radiant forces. The 

 problem is susceptible of two divisions — the absolute and the relative 

 measurement of light. 



I. Given a luminous beam, we may require to express its intensity 

 by some absolute term having reference to a standard obtained at some 

 previous time, and capable of being reproduced with accuracy at any time 

 and at any part of the globe. Possibly two such standards would be 

 necessary, differing greatly in value, so that the space between them might 

 be subdivided into a definite number of equal parts ; or the same result 

 might perhaps be obtained by the well-known device of varying the appa- 

 rent intensity of the standard light by increasing and diminishing its dis- 

 tance from the instrument. 



II. The standard of comparison, instead of being obtained once for all, 

 like the zero- and boiling-points of a thermometer, may be compared 

 separately at each observation ; and the problem then becomes somewhat 

 simplified into the determination of the relative intensities of two sources 

 of light. 



The absolute method is of course the most desirable ; but as the pre- 



* Eead December 17, 1868: see Abstract, antea. p. 1G6. 



