464 PROFESSOR LUDWIG MOSER ON INVISIBLE LIGHT. 
sarily follow that those parts of a body which to our eye appear 
strongly marked in daylight should be very clearly visible in a 
picture produced by its own peculiar rays. 
4. Finally, I will mention the method by which we may con- 
vince ourselves of the action of visible light on many bodies, a 
method which we should be able to employ in all cases, were it 
not that we are compelled to use light of great intensity, whereby 
heat often exerts a disturbing influence. I direct a small camera 
obscura with a lens of 15™™ aperture towards the sun, and insert 
into it a plate of mirror-glass to receive the image. After the 
sun has passed through, I remove the plate and breathe on it; 
a distinct and accurate picture of the sun’s path is produced. 
I have made the same experiment with plates of silver, gold, 
copper, German silver, iron, steel, brass, zinc, and also with the 
compound plate mentioned in section 2, and always with the 
same success. Vapours of mercury, iodine, &c. may be employed 
instead of those of water. 
In order to meet the objection which might be raised, viz. that 
heat contributed to the success of these experiments, I allowed 
the image of the sun to pass through a yellow, bright red, and 
a tolerably bright violet glass, and then to fall on a plate of silver. 
When it was introduced into the vapours the path of the sun 
was visible, though not very perfectly, on the spot where the 
yellow image had acted. The violet image had produced a ~ 
very visible effect, and the red none at all. As I had expected 
this, I had placed the red glass in the most favourable position, 
viz. in the axis of the lens. Another time I allowed the sun’s 
image to pass through a red and a blue glass, each of which 
occupied half of the field of view. The image formed under the 
blue glass was very evident, but could not be traced under the 
red one. It will be seen that these results agree completely with 
what has previously been said respecting the action of the dif- 
ferently coloured rays; but they do not agree with the trans- 
mission of heat. In order to determine this approximately I con- 
structed a small battery of German silver and iron, as proposed 
by Poggendorff, which was very sensitive, although it required 
some time for the needle to adjust itself, a circumstance which 
induced me to regard merely the direction of the deviation. 
When the operation was conducted in the sun it was found that 
the red glass transmitted most heat, and yet it had not rendered 
the image of the sun visible. 
