424 PROFESSOR LUDWIG MOSER ON VISION, 
ber; but we soon find that this is not the most advantageous 
method. No well-defined results are obtained, and we always 
remain in uncertainty, at least as far as regards the more deli- 
cate phenomena. ‘The method of Daguerre is the only one 
which can be advantageously adopted. The operations pro- 
posed by him are calculated to produce a perfect artistical repre- 
sentation, and they answer their purpose effectively, except in- 
deed in the covering the silver with iodine, for the performance 
of which he proposes a very inconvenient apparatus. 
When engaged in physical experiments we may sometimes 
neglect, to a certain extent, some of the minuter prescriptions ; 
but still it is always better to keep constantly in view the pro- 
duction of perfect images. It is not my intention to enter fur- 
ther into this subject at present; I will only describe a con- 
venient apparatus for iodizing, so that others may easily repeat 
the experiments about to. be described. 
A box, of any material, is made of a sufficient size to receive 
the silver plates, and about an inch in height. On the bottom 
of this box a glass plate, covered with any kind of woollen stuff, 
is fixed, and another glass plate is arranged so as to cover this. 
Iodine is sprinkled between these two glasses, and soon diffuses 
itself through the whole of the woollen stuff, and colours it black ; 
the plates prevent its evaporation, and it is very seldom neces- 
sary to repeat the sprinkling. If the ,silver plate be now laid 
above the lower glass plate, a perfectly uniform layer of golden- 
yellow iodide of silver is obtained, and what is of very great im- 
portance, in a space of time which does not vary, unless indeed 
there be very great changes in the temperature. In my appa- 
ratus the operation lasts from 60 to 70 seconds. 
It has been said in Paris that metallic edges are necessary 
for obtaining a uniform layer of iodide; but this is just as in- 
comprehensible as it is erroneous: the silver plate may be ex- 
posed to the vapours of iodine, either by itself or in any other 
way the operator may think fit. 
For many experiments it is desirable to increase the sensibility 
of the iodide of silver, and the chloride of iodine seems to be 
best suited for this purpose. If prepared and used in the man- 
ner I am about to describe, the objections which have been made 
to it on the score of its insecurity, will be found quite invalid. 
A small quantity of iodine is put into a wide-mouthed glass- 
stoppered bottle, and this is inserted open into a larger jar which 
