528 Progress in Science. ‘October, 
revolution; it is seen several times by each spark, but each time in the same 
position. There is no apparent multiplication of a moving object of any sort. 
The interval between the successive instants of visibility was measured 
roughly as follows :—A twning-fork, making g2} vibrations per second, was 
adjusted so as to record its motion upon the smoked surface of a revolving 
cylinder, and an electro-magnet was so arranged as to record any motion of 
its armature upon the trace of the fork; a key connected with this magnet was 
in the hands of the observer. An assistant turned the machine slowly, so as 
to produce a spark once in two or three seconds, while the observer manipu- 
lated the key. In my own case, the mean of a dozen experiments gave 0°22" 
as the interval between the first and second seeing of the cross upon the screen, 
separate results varying from 0°17" to 0°39". Another observer found 0 24" as 
the result of a similar series. Whatever the true explanation may turn out 
to be, the phenomenon at least suggests the idea of a reflection of the nervous 
impulse at the nerve extremities, as if the intense impression upon the retina, 
after being the first time propagated to the brain, was there reflected, returned 
to the retina, and from the retina—travelling again to the brain—renewed the 
sensation. I have ventured to call the phenomenon ‘recurrent vision.’ ”’ 
Investigations to test the influence of pressure on the spectra of gases have 
been carried on by M. Cailletet. He fixed two platinum wires in the end of a 
thick glass tube, into which the gases were passed. ‘The spark from an in- 
duction coil connected with three Bunsen elements passed between the wires. 
At ordinary pressure the bright lines of the spectra of the gases appeared on 
a slightly illuminated ground, and as pressure was increased they grew 
brighter; but they by-and-bye became merged ina continuous spectrum, whose 
brightness also increased with the pressure. At a certain pressure (between 
forty and fifty atmospheres) the discharge suddenly ceased; and though the 
battery power was increased, and the distance between the platinum wires 
reduced to one-half millimetre, it was not possible to obtain the spark beyond 
this point. It is thus seen that a spark which passes easily in the rarefied 
gas of Geissler tubes. or the electric egg, meets with considerable resistance 
in compressed gas. The brightness of the spark at the point beyond which 
the discharge is unobtainable is 200 times greater than at ordinary pressure. 
Dr. Burt’s investigations upon the subje& of the influence of variously 
coloured light upon vegetation shows that green light is almost as fatal to 
vegetation as darkness; that red light is very detrimental to plants, although 
in a less degree than green light; that although yellow light is far less detri- 
mental than the preceding, it is more injurious than blue light; and that all 
the colours taken singly are injurious to plants, and that their union in the 
proportion to form white light is necessary for healthy growth. 
M. P. Thomas, acting under instructions from the Paris Society of Civil 
Engineers, has reported upon the process of the oxygen light of Tessie du 
Motay. The report siniply treats of technical advantages and disadvantages, 
leaving out of sight the economical question, which is somewhat to be 
regretted, in view of the indistin@ statement of the causes which have led to 
its removal from some of the streets of Paris where it had been introduced. 
The conclusions arrived at are the following :—1. Theoretically, the combus- 
tion of oxygen does not increase the illuminating power of a given volume of 
SENS He Practically, however, it enables a burner to consume four times the 
quantity of gas that can be burned in air, without detriment to the utilisation 
of the light which may be developed. In particular, it utilises the entire 
luminous capacity of the gases, however rich, and in almost any quantity. 
Consequently it would be disadvantageous to employ it for ordinary street- 
lighting, on account of the limited quantity of gas consumed by the burners, 
the only advantage gained being the beauty of the light, provided the gas is 
very rich. Here, unquestionably, would arise the objection of expense, from 
the complication of the apparatus. But it is very advantageous, and the more 
so in dire@ proportion to the richness of the gases employed, for great centres 
of light (sun-burners, &c.), where a large volume of gas is to be consumed 
without loss. 
