166 
CAPTAIN ABNEY ON THE SENSITIVENESS 
Some twenty-four series of observations with the spectram were made with discs of 
different sizes, and all gave similar results to those tabulated, and hence we may 
fairly take it as proved that whatever the size of illuminated disc may be, all rays 
follow the standard curve of extinction, that is to say, that if with a disc of one size 
a green ray has to be reduced to xo-^olTo of if® original intensity to fail to stimulate 
the retina to cause a sensation of light, and with another smaller disc observed at the 
same distance to only soooo’j every ray in the spectrum, when illuminating the 
same size of discs, will have to be reduced in the same proportion. 
6. Hie most Sensitive Condition of the Retina. 
In all observations of extinction, the greatest care must be taken to obtain the 
most sensitive condition of the eye, and I have found that it is useless to attempt any 
readings until the eye has been placed in darkness for at least twelve minutes. It is 
well to vary the plan of reading by first reducing the light till it disappears, and then 
to note the exact point when it reappears. The latter would be the best plan to use 
entirely, since the eye is in darkness till the very last part of the observation, were it 
not for the fact that, in noting the reappearance of the light, we are noting somethino’ 
less defined than when we are noting its disappearance. The combination of the two 
methods has been found to be the most satisfactory plan. A very great difficulty 
also often arises in keeping the axis of the eye employed in a line with the object 
observed, particularly when it is of very small angular aperture. It has been found 
advantageous to have a feeble liglit which can be flashed momentarily on the aperture 
to guide the eye just previous to the disappearance of the light. This is more 
particularly the case when it is desired to extinguish with the centre of the retina, 
as is the case in these observations. The intrinsic light in the eye is also sometimes 
difficult to deal with, but by judiciously recognizing the fact that it exists, and by 
giving proper intervals between each observation, one is enabled to surmount this 
difficulty. Again, a healthy state of mind and body is most essential when making 
observations, as the sensitiveness of the eye largely depends on it, 
7. Laiv Connecting the Angular A’perture with the Extinction. 
The next investigation carried out, was to ascertain if any law connected the 
angular aperture of the object observed with the diminution of the intensity of the 
light which was required to cause invisibility. For the purpose, a large number of 
diaphragms of very difiering apertures were inserted in front of the ground glass 
(fig. 3). For the sake of plotting, in the first instance, and as they give the most 
rational scale, the diameters of the discs were expressed in powers of 2, thus inch, 
which is 2“\ is used on the scale of abscissae as — 1 ; as — 2, and so on—all 
diameters not being expressed in exact powers of 2 being calculated out in the 
ordinary way. 
