396 Progress in Science. [July, 
rendered possible, for by heating the gas to redness the sulphur combines 
with hydrogen to form sulphuretted hydrogen, which can be easily removed 
by passing through a purifier containing oxide of iron. By passing coal-gas, 
containing 30 grains of sulphur in too cubic feet, through a red-hot tube, and 
then through an iron purifier, the sulphur was reduced to about 5 or 6 grains 
in roo cubic feet. It might be imagined that the passage of the coal-gas 
through a red-hot tube would deteriorate its quality ; but Mr. Harcourt found 
the contrary to be the case. By passing gas of 14'g1 candles rapidly through 
a tube heated to dull redness, the illuminating power was found to be 15°1 
candles, and by passing the gas through a tube heated to bright redness 
the illuminating power was increased to 16°66 candles. A parallel case to 
this occurs when marsh-gas is decomposed eudiometrically into hydrogen 
and carbon; the resultant gas occupies almost twice the original volume of 
the gas, but possesses a far greater illuminating power than that of the 
original marsh-gas, owing to the presence of ;acetylene or some such body. 
It seems easy to deduce from Mr. Harcourt’s results a process for the 
desulphuration of coal-gas. 
In reference to the very simple and ingenious arrangement for a vertical 
lantern made by Mr. Ferguson, and described in our last issue, we would 
add a further suggestion made by Prof. Morton, President of the Stevens Insti- 
tute of Technology, whose form of the same apparatus we published in our last 
October number. This is to carry yet a step further the simplicity of the 
instrument by substituting a watch-glass full of water for the photographic 
or other objective lens. Dr. Morton also tells us that he finds this method can 
be developed into a very striking illustration of refraction and the action of lenses 
in the following manner :—A well-defined photograph on glass is placed as an 
object in the vertical lantern, the place of the objective being supplied by an 
empty watch-glass previously adjusted. There will then appear on the screen 
no image, but only a nebulous mass of light. On pouring water into the watch- 
glass, however, a sharply-defined image will come out as soon as the water 
ceases to move in ripples. By removing the water and substituting alcohol 
or some more highly refracting liquid, such as a solution of muriate of tin, 
a lens of shorter focus and higher magnifying power will be obtained. 
Dr. Morton sends us also the following explanation of the general relation 
between the condenser and objective in the magic lantern, on which was 
founded the plan first adopted by him of separating the ordinary lantern condenser 
into two elements, one of which was placed before and the other after the 
mirror of the vertical lantern, by which the efficiency of that instrument was so 
largely increased. It is curious to note that this point seems to have escaped 
the able constructor, M. Duboscq, who publishes in Les Mondes, vol. xxiv., 
p- 649, a vertical lantern in which this very important element is absent. “If 
a system of condensers are well arranged, the front surface of the outside 
lens will be evenly illuminated, as may be seen by placing a sheet of thin 
paper against it, but from this surface the rays will travel outwards with alk 
the irregularities of distribution caused by spherical and chromatic abberration, 
and by the action of the condensers as image-producer of the source of light.” 
“If now the objeét-glass while in focus with the obje@, is also in focus with 
the front of the condenser, it will produce on the screen an image of the 
equally illuminated surface of the condenser, the irregularities in direction of 
the rays as they leave that surface having, as we know from the simple theory 
of image formation by lenses, no effect inimical to such a result. But if the 
object is moved forward on the cone of rays issuing from the condenser, then, 
when the obje@-glass is in focus with this, it will tend to produce an image 
of a section of the light cone made in the same plane.” Here, however, the 
irregularities in distribution of the light before noticed have developed to the 
extent of rendering the area of such a cross section very irregular in illumination 
and colour, and this irregularly-illuminated surface the object-glass will 
reproduce in the image which it throws upon the screen. ‘If in the vertical 
lantern we place the condensers all together before the first mirror, then the 
object above the mirror will be at a considerable distance from the front of 
the condensers, and the ill results already named will follow. If, however, 
