TERRESTRIAL REFRACTION AND MIRAGE, 
BY 
MAJOR P. A. MACMAHON, R.A., F.R.S. 
(A Lecture delivered at the Royal Artillery Institution, Woolwich, 7th February, 1895.) 
Cotonen C. Trencnu, R.A., In THE CHAIR. 
Tue Cuarrman—Major MacMahon is kind enough to give us a lecture 
to-night ; itis not necessary for me to introduce him to you (applause). 
Masor P. A. MacManoy, F.R.S.—Colonel Trench and gentlemen, 
the reason why I undertook this short investigation into Terrestrial Re- 
fraction was that a number of inquiries had reached me in regard to the 
effect which terrestrial refraction might have upon artillery fire. I 
think, on the whole, the result of the investigation is to show that the 
facts which are available are of a distinctly reassuring character and 
this I hope will appear as I proceed. 
For the benefit of those who are not very familiar with the subject I will 
give a short definition of refraction. When a ray of light passes from 
point to point of a medium which is everywhere similarly constituted 
its path is a straight line; but if it passes from one medium to another 
which is differently constituted or is of a different nature, then, 
in general, the ray of ght will be bent at the surface common to the 
two media; and this bending is termed the refraction of light. There are 
many familiar examples; one of the most familiar is the bent appearance 
of an oar in water. If we take the common surface of air and water we 
have a ray of light, which is incident at a certain angle to the normal 
entering the water and passing through it closer to the normal, 
—it, in fact, approaches the normal on entering the water. If we have 
a rod (see Fig. 1) placed in the water the lower portion of the rod H is 
Fie. 1. 
SS = ae = 
seen by the eye placed at C' by the path CD, DH, there being a bending 
6, VOL, XXII, 
