on the terrestrial Rays that occasion Heal. 513 



red rays ; then, if the 86 rays of light which this glass stops, 

 are supposed to be all of that sort, the heat which will be 

 stopped in consequence, will, according to the experiment we 

 have mentioned, amount to 86 multiplied by ,375, that is, 32 

 rays of heat ; but, since 250 have been stopped, there will 

 remain 218 to be accounted for. 



In this calculation, a manifest concession has been made, 

 which ought to be explained. When I mention 86 red-coloured 

 or red-making rays, I mean so many of them as will make up 

 86-thousandths of the whole effect of light ; for the quantity 

 of heat and light transmitted, or stopped, in all the experiments 

 that have been given, has been reduced to what proportion it 

 bears to unity ; and, having afterwards represented the joint 

 effect of every ray of heat and light by 1000, each mean ray 

 of heat must be the thousandth part of that effect; but, a mean 

 ray of light, although it be likewise the thousandth part of the 

 whole effect of light, will not be so of heat, because the whole 

 effect of the latter is partly owing to rays that have been proved 

 to be invisible. On this account, the 86 mean rays of red light, 

 stopped by our bluish-white glass, cannot even amount to a 

 stoppage of 32 rays of heat, which we have allowed. 



As I have made the concession on one hand, I must explain 

 an advantage that may be claimed on the other ; which is, that 

 mean rays and promiscuous ones have already, in a former 

 Paper, been proved to differ considerably, and that it remains 

 therefore unknown how many red-making rays we may sup- 

 pose to be stopped, in order to make up 86 mean rays of light. 

 In answer to this, however, I must observe, that the number of 

 promiscuous rays of light and of heat must always be inversely 

 as their power of occasioning those sensations ; so that if, for 



