( 739 ) 
The colour value. Seeing that side by side with the subhaloid we 
always have the coloured halogen, although in a less than equivalent 
quantity, the colours of the subhaloids will never appear so clearly 
as in the case of the preparations prepared chemically by M. Carry 
Lra’s and Lipro-Crammr’s**) method. 
The place of the a-subhaloid. My investigations as to the substance 
of the latent image?*) and its preparation *°®) show that in the case 
of iodide as well as of bromide and chloride the a-subhaloid has a 
green colour. As the assumption of a subhaloid still richer in halogen 
to account for Bunsrn-Roscon’s photo-chemical induction *') and for 
auto-sensitation **) is superfluous, I have attempted to trace this 
possibly extant subhaloid by the photo-chemical method indicated. 
If silver bromide with an excess of potassium bromide is made to 
precipitate from a silver nitrate solution, dried, and exposed under 
carbon tetrachloride to moderately strong daylight, it soon begins to 
show a green colour. In this preparation the diffusion of the liberated 
halogen is extremely slight, and potassium bromide is a very weak 
chemical sensitizer, with which the reaction product KBr, with the 
subhaloid partially shows a reversed reaction in the dark. Under 
these circumstances the e-subbromide is, therefore, obtained photo- 
chemically in visible quantities, and hitherto this has been the simplest 
method found by me of preparing light-proof preparations from this 
substance. The halogen pressure can, however, be increased still 
more by omitting this weak chemical sensitation as well. In a dry, 
pure collodion coating pure silver bromide yielded a grey discoloration 
with the lowest light intensity with which decomposition is still 
to be observed. In the dark the light yellow silver bromide appeared 
again after some time; consequently an almost complete re-formation 
of the silver bromide takes place, from which it may be inferred 
that the loss of bromine has been reduced to a minimum, and the 
subhaloid richest in halogen may have appeared with the photo- 
chemical decomposition in a visible quantity. But even now the grey 
colour need. not be attributed to a subhaloid richer in halogen than 
the a-subhaloid, for the equivalent amount of bromium may mix its 
brownish red colours with the green of the «-subbromide so as to 
yield grey. As, therefore, the green a-subhaloid is to be considered 
the one richest in halogen we know at present, we may say that 
every other colour must be ascribed to a subhaloid poorer in halogen. 
Use of the a subhaloid. If in these experiments we start from the 
green a-subhaloid instead of from silver haloid, a different sequence 
of colours is obtained. With silver nitrate the green preparation very 
rapidly turns grey or black through the formation of mixed colours 
