4.4. Dr. Woods on a New Actinometer. 
positively whether such a double action does take place; it ap- 
pears probable ; but it might be, that as long as the light con- 
tinues to fall on the solution, the saturated state is kept up, and 
consequently none comes off, except what is at the moment pro- 
duced. This point will require some trials to decide. The extent 
to which the rise occurs is, however, not great, as may be seen 
by referring to p. 42, and might have been due in this case to 
an accidental circumstance. 
If I had time to make experiments with this instrument, I 
would not publish this account of it, as the doubts I have ex- 
pressed might be solved, and more certain results might also be 
obtained by using tubes of various diameters, until the most 
proper for all purposes would be found: but I have done all I 
expect to do for an uncertain period; I therefore give the de- 
scription of the actinometer as it is, and for what it is worth, to 
the scientific world, believing it to be at least the germ of a 
useful and interesting instrument. It may be of use to the 
photographer as a means of exactly measuring the time of expo- 
sure of a sensitive plate. The period of time, as reckoned by 
seconds, will not always give the same amount of actinic force, 
as the light may vary considerably between two experiments, and 
yet not affect the eye. If, however, a good picture be obtained 
during the rise of, say two degrees on the actinometer, the same 
amount of actinism must always be present during the same rise, 
be the time of rising longer or shorter. To science also it ought 
to be a valuable help if its indications are sufficiently reliable. 
I should have mentioned that the strength of the solution of 
peroxalate of iron I employed was 35 grains to the ounce of 
water; but I believe this strength might be advantageously in- 
creased. It will also be necessary, for comparative experiments, 
to have a cover for the actinometer, in which an aperture is cut 
of a certain size, say one or two square inches, in order that a 
known extent of surface may be always acted on. The tube will 
also require to be covered in delicate experiments, as the light 
acts on the fluid in it, as well as on that in the bottle. 
I must advise those persons who adopt the rough and ready 
method of manipulating, that in making this apparatus, simple 
as it may appear, there is great caution to be observed in causing 
the stopper of the bottle to be air-tight, and also the apertures 
through which the tubes and thermometer pass. When the fluid 
rises in the tube, a great pressure is sustained by the interior 
of the phial; and if this be not thoroughly provided against, 
the air will find some small hole, too minute for observation, by 
which, very gradually, almost insensibly, to escape ; and so the 
results would be vitiated. 
Parsonstown, Dec. 10, 1859. 
