Hf. Draper— Oxygen in the Sun. 265 
As to the induction coil, it is only needful to say that it gives 
a good thick spark, which is limited to twelve inches to avoid 
the risk of injuring the insulation. The Leyden jars are four- 
teen in number, having altogether seven square feet of coating 
on each surface. 
The arrangement of the terminals from the Leyden jars to 
get the steadiest and brightest effect has offered great difficulties. 
The condensed spark taken in the open air or ina gas under 
atmospheric pressure pursues, if unconfined, a zigzag course, 
and this is apt to produce a widening of the lines in the photo- 
graphed spectrum. But, after many experiments, it turned out 
that the spark might be compressed between two plates of thick 
glass, or, better yet, between two plates of soapstone. If the 
interval between the plates was directed toward the slit of the 
spectroscope the lateral flickering of the spark was prevented, 
and yet at the same time the spark was freely exposed to the 
slit without the intervention of glass or any substanee on which 
the volatilized metal from the terminals could deposit. Very 
took place in the capillary portion, and partly because the ter- 
minals became so hot as to melt and crack the glass. More- 
I have tried the effect of warming the air by passing it through 
a coil of brass tube maintained at a bright red heat, but this 
pressor. 
The optical part of my apparatus has undergone many modi- 
- first a Hofmann direct-vision prism was combined 
With aléns of six inches focus; this was soon after replaced by 
a Browning direct-vision prism and a lens of eighteen inches 
er 
x inches focus were employed. nea d 
this winter, consists of a collimator of two inches a ye 0 
twenty-six inches focus, succeeded by two bisulp ogi haemt 
hotograph- 
prisms of two inches aperture and an observing or 
ing lens of six feet six inches focal length. These prisms 
belong to Mr. Rutherfurd and are the same he made for produ- 
cing his celebrated solar prismatic spectrum. This gives a 
dispersion of about eight inches between G and H and enables 
