8 REPORT—1857. 
operation of depositing it is so easy and prompt, that it can easily be repeated. To 
resume, the new instrument, compared with the refracting telescopes, gives, at much 
less cost, more light, more distinctness, and is free, like the reflecting telescope, from 
all aberration of refrangibility. 
On the Colour of Salts in Solution, each Constituent of which is coloured. 
By J. H. Gravstone, Ph.D., F.R.S. 
It is a general law, that “ all the compounds of a particular base or acid, when in 
aqueous solution, absorb the same rays of light; ”’ hence it may be deduced, that 
when a coloured base and a coloured acid combine, the resulting salt will transmit 
only those rays which are not absorbed by either constituent, or in other words, 
only those rays which are transmitted by both. This was proved to be actually the 
case by a prismatic examination of compounds of chromic, permanganic, and car- 
bazotic acids with copper, iron, nickel, uranium, and chromium. Though the 
compounds of chlorine, bromine, and iodine with hydrogen and most metals are 
colourless, the compounds of these halogens with gold, platinum, and palladium 
exhibit an absorption of light due to the halogen as well as that due to the metal. 
The same is true in respect to chlorides, bromides, and iodides of copper, iron, 
nickel, and cobalt, when these salts are dissolved in a minimum of water; but 
when more water is added, the colour changes, and the absorption due to the halogen 
no longer exists. In one or two of the cases examined a slight variation from the 
general law occurred; and ferrocyanide of iron forms a complete exception. The 
double chloride of platinum and copper shows the absorbent effect of all three 
constituents. 
On the Effects of Heat on the Colour of Dissolved Salts. 
By J. H. Grapstonr, Ph.D., F.RS. 
If a coloured salt be dissolved in water, heating the solution does not usually 
affect the colour of it. In not a few cases, however, the colour is rendered more 
intense, and altered somewhat in its character. Among the examples mentioned 
were ferridcyanide of potassium, meconate of iron, chloride and bromide of palla- 
dium. In other cases, heating the solution produces apparently a total change of 
colour: for instance, chloride of copper passes when heated from blue to green ; 
chloride of nickel from a bluish to a yellowish green; sulphocyanide of cobalt, or 
chloride of cobalt dissolved in aqueous alcohol, from a pale red to a deep bluish 
purple. In all these instances heat causes the absorption of a larger quantity of 
rays by the solution; but this appears to depend sometimes upon some purely 
physical cause, at other times upon some chemical change. With ferridcyanide of 
potassium, and similar salts, a certain thickness of the heated solution produces 
precisely the same effect on the spectrum as an increased thickness of the same 
solution when cold. With chloride of copper, and similar salts, the somewhat 
diiute solution when heated produces the same effect on the spectrum as the same 
solution when concentrated and cold,—these salts being all of that character which 
is altered in colour by the addition of water. 
On Improvements in the Optical Details of Reflecting Telescopes and 
Equatoreal Instruments. By Tuomas Gruse, M.R.I.A., Dublin. 
The author stated, that while the Earl of Rosse, by his achievements, had placed 
beyond doubt the practicability of producing specula for reflecting telescopes at 
once as perfect as could be desired, and as large as could be made practically useful, 
the achromatic object-glass had received but little increment of size; and though 
the Messrs. Chance, of Birmingham, had produced a pair of discs, of optical glass, 
of 29 inches diameter, yet these had been allowed to be transferred to another 
country, where the work of forming them into an object-glass was still to be effected, 
Four years had now elapsed since the production of these discs, and the refracting 
telescope may now be considered as being completely distanced in size by its com- 
petitor, the reflector. Under such circumstances, it was important, he conceived, 
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