7 Monthl ical 
298 Transactions of the See ee eee 
There seems, then, good ground for concluding that in all cases 
the conditions of least aberration should be applied. 
A ray of light may be of any colour, and obeys exactly the same 
laws as every other. Spherical aberration is equally true of 
coloured rays as of light, considered theoretically homogeneous. 
A violet or a red ray obeys the law of the triangle. 
Regard now this rough sketch of gold leaf viewed under a high 
power by transmitted solar rays. The thing itself is full of mystery 
and beauty. Why are here red spots, bright spots, and black— 
inky black—edgings? The wave theory of light may give us a 
clue. Whence comes the rich, translucent, malachite green of the 
gold ? Whence the red and whence the'black ? Science shakes her 
head, and says the answer is long and deep. Ask the rainbow and 
the dewdrop whence come its colours, or the rose-leaf. Absorption, 
reflexion, refraction, undulation, vibration of the ether and rays of 
interference. Such are the hard dry answers to these simple 
questions. 
But here all definition gets its programme of sharpness and 
decision. 
What is this black edging? It is almost specific of a fine 
definition. 
What the red spots? The gold is thicker there, perhaps, but 
I am not sure of that. The best leaf is only tscoooth thick. 
Again, view these coarse disks. The same black rings; the 
light is dead, producing darkness; wave collisions wave. 
Sir John Herschel describes some of the effects of interference 
and diffraction, as producing some of the most gorgeous pheno- 
mena in the range of physical science. 
But definition is supremely dependent upon the extent, nature, 
and direction of these collisions and decussations. 
It may now be stated in general terms that :— 
If an object composed of minute bodies be placed in a blaze of 
confused decussation, fine definition is impossible. 
If there be any confused decussation in the focal rays of the 
object, as a brilliant point of light, however small, i acquires a 
spurious exaggerated disk, more or less colowred. 
These cardinal facts appear to sum up the whole theory of 
illumination and definition. 
An image of an object is an assemblage of the images of every 
point in the object.* 
If every poimt produces a spurious disk, the final image is 
simply an agglomeration of spurious disks: and the definition is 
better or worse, according as these disks are smaller or larger. 
* See Parkinson’s ‘ Optics.’ 
