290 



Dr, J. Hopkinson on the 



[June 14, 



blood-vessels, but prominently absent from the collecting lymphatic 

 channels lying alongside of these vessels, thus supporting the hypothesis 

 they formerly emitted, that these cells were merely links in a nutritive 

 chain, not radicles of the lymphatics, even when, as in tendon, the cornea, 

 &c, they are connected with the lymphatics. The paper is illustrated by 

 about a dozen and a half of camera-lucida drawings of microscopical 

 specimens in their possession. 



II. " Refractive Indices of Glass/' By J. Hopkinson, D.Sc, 

 M.A. Communicated by Professor G. G. Stokes, Sec. R.S. 

 Received May 25, 1877. 



Most of the following determinations were made two years ago. 

 They were not published at once, because the results showed more 

 variation than was expected. They are now made known for two reasons. 

 First, most of the glasses examined are articles of commerce, and can be 

 readily obtained by any person experimenting upon the physical properties 

 of glass ; these glasses only vary within narrow limits, and their variations 

 may be approximately allowed for by a knowledge of their density. 

 Second, most of the prisms having three angles from each of which 

 determinations were made, the probable error of the mean is very small, 

 and any error of the nature of a blunder is certainly detected. 



The form in which to present these results was a matter of much 

 consideration. A curve giving the refractive indices directly is unsuit- 

 able, for the errors of observation are less than the errors of curve-drawing 

 would be. The theory of dispersion is not in a position to furnish a 

 satisfactory rational formula. The most frequently used empirical for- 

 mula is fj = a + b ~ + ., where ix is the wave-length of the ray to 

 which fx refers. But to bring this within errors of observation it is 

 necessary to include -i, which appears to be almost as important a term 



as -i. There are two points of importance in the selection of an empi- 



rical form : first, it must accurately represent the facts with the use of 

 the fewest arbitrary parameters ; second, it must be practically convenient 

 for the purposes for which the results are useful. 

 In the present case the most convenient form is 

 Li-l = a{l + bx(l+coc)}, 

 where x is a numerical name for the definite ray of which jj. is the refrac- 

 tive index. In the present paper line F, being intermediate between the 

 strongest luminous and chemical rays, is taken as zero. Four glasses, 

 Hard Crown, Soft Crown, Light Flint, and Dense Flint, are selected on 

 account of the good accord of the results, and the mean of their refractive 

 indices Jl is ascertained for each ray ; this is taken as a standard scale in 

 which t T=ju — 



