31 
This was the beginning of the application of photography 
to the heavenly bodies. Photography, and particularly the 
application of the dry plate to the telescope, had done as much 
to advance astronomy as the invention of the spectroscope 
and the telescope. The head of the comet, said Father 
Cortie, had been analysed by the spectroscope, by which it 
was seen that the material of the comet gave out radia- 
tion, which, when analysed by the spectroscope gave out 
yellow, blue, and dark-blue images. If compared with spectrums 
of hydro-carbon, they saw that the head of the comet was 
formed of hydro-carbons. The spectrum of a comet was 
the one given by the base of an ordinary gas flame. If the 
comet approached the sun, other lines began to appear in the 
spectroscope which betokened the existence of sodium and 
iron. Examined by the polariscope, they found that a great 
deal of the tail was reflected sunlight. Again referring to 
the repulsive force, the lecturer said that the pressure of light 
upon small particles had been demonstrated with a sandglass, 
which, when a strong beam of light was directed upon it, 
allowed the heavy particles to fall, but drove back the lighter. 
Instances of the breaking up of the tail and the formation 
of fresh comets from these, were given, the lecturer attributing 
this to the comet having met with some resisting medium 
which shattered the tail. The theory of the pressure of light 
would account for most of the phenomena of comets’ tails. 
The limits in which the pressure of light could act were two 
and a half times the wave length of light and it was only 
within those limits that the light pressure overcame the 
attraction of gravitation. In conclusion, Father Cortie said 
that the last word of science had not been said on the subject ; 
it was only in popular literature, and when men wished to 
throw stones at religion, that they found dogmatic statements 
made with regard to science. 
The lecturer, in reply to questions and a hearty vote of 
thanks, said the luminosity of the comet was a great problem. 
A great parHofjfthe light of the comet was reflected. Some 
said it was due to phosphorescence, some to collision of the con- 
stituent particles. They knew that part of the light was due to 
reflected sunlight, but what part was due to phosphorescence 
or collision they did not know. Comets were unattached, 
a kind of no man’s land. As Christians, they all acknowledged 
as first cause God Almighty. He knew that Haeckel and other 
Germans denied the first cause, but they knew that one 
cause followed another, and the further back they went they 
came ultimately to matter and motion. Matter, however, 
could not create itself and therefore they came to Almighty 
God. 
