of Edinburgh, Session 1869-70. 
179 
duction, and rests on experience, is an analytic judgment, that it 
can be reached by a purely formal dividing and compounding of 
the definitions of terms. Such a proposition could be shown to he 
true without any figure or any experiment. Yet the proposition 
is, we are told, involved in the notions ; we cannot know what 
lines, angles, &c., are without knowing this too. If this means 
anything, it means that Euc. I. 4 is a synthetic judgment a ‘priori ; 
and that, after all, Kant and the mathematicians are right, and Mr 
Mill and the empirical logicians wrong. 
4. A Simple Mode of Approximating to the Wave-Length 
of Light. By W. Leitch, Assistant to the Professor of 
Natural Philosophy in the University of Glasgow. Com- 
municated by Professor Tait. 
The fundamental phenomenon or fact of the science of optics is 
vision, that is, the perception we have of distant objects through 
the eye, or by the sense of sight. That vision is an effect trans- 
mitted to the mind by the object seen, is a necessary truth, involved 
in the definition of the term, and independent of all theoretical 
views beyond the consciousness of that perception. 
Common observation informs us that vision cannot take place 
without that which we call light, and that light itself cannot exist 
without the presence of a self-luminous body. Every one has a 
distinct conception of the meaning of the' terms light and luminous; 
their definition according to that conception would be a verbal 
exercise of no utility at present. 
Next may be placed the fact, first revealed by astronomical ob- 
servations, and afterwards verified by other experiments, that light is 
not transmitted instantaneously, — in other words, that some portion 
of time elapses between the occurrence of a visible phenomenon 
and our perception of it by the eye, such as, for simplicity, the 
passage of an electric spark, or the occultation of a star by the dark 
body of the moon or of a planet; and that the portion of time in 
question is in direct proportion to the distance of the object seen 
from the eye, the intervening medium being the same. 
The progressive motion of light from the object seen to the eye 
being established, and the supposition that it is a substance emanat- 
