468 Ignoramus et Ignorabimus. [August, 
eye does not readt to all the solar radiations. At both ex- 
tremities of the spedtrum there are rays, invisible to us, the 
ultra-red and the ultra-violet. We can demonstrate their 
existence by certain artifices. It is highly probable that 
they — at least the ultra-violet — produce impressions on the 
eyes of certain insedts, but so far as they are concerned we 
are blind. Now if those rays affedted our eyes it may be 
regarded as certain that we should see objedts not precisely 
as we now see them, but more or less modified. This will 
appear the more manifest if we refledt that fully one-third 
of the solar radiations do not adt upon our retina at all. 
This consideration not unnaturally connedts itself with 
the question of colour-blindness, which has been so ably 
examined in our columns by Mr. Jabez Hogg. Is this 
defedt — which in the sufferer very much narrows, or, at all 
events, renders less precise the information received through 
the light-sense — increasing or decreasing in frequency ? In 
the latter case, is it not at least conceivable that a time may 
come when the sensibility of the retina shall be extended ? 
But besides the quality of the solar radiations, then- 
quantity also fixes a limit not ordinarily passed, to our power 
of sight. Many animals can see where we cannot see at 
all, or see only with the greatest difficulty. 
It will perhaps be contended that an inability to recognise 
very feeble lights can make little difference to our insight 
into the phenomena of Nature, since if objedts are invisible 
to us in the night we may revisit them in the day, or we 
may make use of artificial lights. But what if, as certain 
fadts lead us to suspedt, numbers of plants and animals 
emit phosphorescent light, too faint for us to recognise ! 
Here a greater acuteness of sight would open to us quite a 
new world. 
Passing from sight to hearing, we again meet with evi- 
dence of the imperfedtion of our senses, and the consequent 
incompleteness of their teachings. There are sounds too 
deep to impress our auditory nerves, and others again too 
acute. Of the existence of the latter, at least, we are con- 
vinced by the varying range of the sound-sense in different 
individuals, whilst certain insedts appear affedted by vibra- 
tions much more rapid than any which to us produce an 
audible sound. Hence in pronouncing any animal species 
“ mute ” we are possibly in error. It may have the means 
of producing sounds which simply do not fall within our 
compass of hearing. 
Another human shortcoming is our limited ability to de- 
termine the diredtion of a sound and the distance of its 
