16 REPORT—1863. 
nitrogen at a tension of about 50 millimétres, you will, without the Leyden jar, 
get a most beautiful spectrum of the first class. After interposing the jar, a splendid 
spectrum of the second class will be seen. But here the case is more complicated 
still. The above-mentioned spectrum of the first class is‘not a simple one, but it 
is produced by the superposition of two spectra of the same class. Tieetted nitro- 
gen, at the lowest temperature, has a most beautiful colour of gold. When its 
temperature rises, its colour suddenly changes into blue. In the first case, the cor- 
responding spectrum is formed by the less refracted bands extended towards the 
violet part ; in the second case, it is formed by the more refracted band of the spec- 
trum extended towards the red. Nitrogen, therefore, has two spectra of the first 
class and one spectrum of the second class, The final conclusion, therefore, is that 
sulphur has two, nitrogen three, different allotropic states. It may appear very 
strange that a gaseous body may have different allotropic states, z. e. different states 
of molecular equilibrium. It may not appear, perhaps, more strange that a sub- 
stance, hitherto supposed to be anelementary one, may really be decomposed at an 
extremely high temperature. From spectral analysis there cannot be taken any 
objection that sulphur and nitrogen may be decomposed. Chloride of zine (or 
cadmium), for instance, exhibits two different spectra. If heated like sulphur, and 
then ignited by the discharge of Ruhmkorft’s coil, you will get a beautiful spectrum 
either of chlorine or of the metal, if either the Leyden jar be not interposed or be 
interposed. There is, in this case, a dissociation of the elements of the composed 
body in the highest temperature, and recomposition again at a lower temperature. 
You may consider the dissociation as an allotropic state, and, therefore, I may 
make use of this term as long as the decomposition be not proved by the separated 
elements. 
On the Focal Adjustment of the Eye*. By Barnarp §. Proctor. 
The object of the paper was to afford answers to the following questions :— 
Is it occasionally, generally, or universally that the human eye has distinct foci 
for vertical and horizontal lines ? 
Is the power of altering the focus of the eye exceptional or general? and what is 
the extent of the change ? 
Can eyes, having distinct vertical and horizontal foci, be made to adjust these 
foci to any particular distance at the same time ? 
Have the two eyes generally different focal lengths ? : 
Does a good resolving power always accompany a good adjusting power ? 
Are markings on a flat surface resolved better by one eye or both? 
What is the appearance of a vertical line, and what of a horizontal line and a 
point, when within or beyond focus ? 
Do the powers of the eye vary much with time of day, bodily or mental 
fatigue, &c. ? 
For experimenting upon the focus, an object was constructed consisting of a 
darkened glass, upon which were scratched lines so as to transmit light. The 
design adopted was a cross consisting of fine double lines, simple appliances being 
adopted for transmitting through these lines a constant amount of light, and for 
varying the distance between the object and the eye of the observer. For ascer- 
taining the resolving power of the eye, a test-object was constructed in the same 
manner, the design upon it consisting of two lines diverging at a very acute angle 
from a point, and bearing an index showing how many thousandths of an inch 
space there was between the lines at the point where the observer ceased to have 
the power of distinguishing them as two lines. : 
This test-object, and the same mode of using it, was adopted in experimenting 
upon the power of various eyes to change focus: the resolving power having been. 
ascertained for the shortest comfortable focus, the same was tried for double and 
three times the distance. 
In all these experiments, the person whose sight was being examined did not 
know the measurements till the conclusion of the experiments ; thus was ayoided 
* The entire paper is printed in the Philosophical Magazine for October 1863. 
