SCIENTIFIC SUMMARY. 
217 
process of focussing the telescope to a distant object, and the collimator to 
the focus so obtained being only applicable to the mean rays, and useless 
with quartz lenses and ultra-violet undulations. It consists of consecutive 
approximations. If the collimator be out of adjustment, there will be two 
positions of the prism, one on either side of the minimum deviation, which 
will bring the desired ray into the field (a and b). 
In position A the telescope is to be focussed to distinctness ; in position b, 
the collimator. After three or four trials no change of focus is required, 
and the adjustment is made. 
Theoretically, the method requires absolutely plane faces to the prism, 
which are difficult to obtain ; but practically the change of adjustment thus 
required is very small, not amounting to more than one-half per cent, for an 
exceptionally bad specimen. 
The Acoustics of Auditoria. — Mr. W. W. Jacques contributes to the 
Journal of the Franklin Institute some facts of considerable value on this 
neglected subject. 
The first is a laboratory investigation into the effect of currents upon a ray 
of sound ; the second and third studies, by different methods, of the effects of 
•currents of air in a hall or theatre upon the waves of sound. 
At a point a was placed a source of sound, and at e the ear. Between 
these were placed substances heated so as to produce currents of air corre- 
sponding in density to those found in an auditorium. AVith a small leaden 
organ-pipe the intensity was increased, but the distinctness impaired. The 
same effect occurred with a clear voice, accompanied by a sensation of repe- 
tition. On the violin the effect was less, and with a drum nil. 
In the second experiment the sound-waves were actually traced out in 
space, and their confusion consequent on introduced currents of air demon- 
strated. A platform 6 feet wide and 12 feet long was set on end in a room 
92 feet long by 65 feet broad. On one side was a B 4 stopped organ-pipe, 
blown with a constant pressure of air. 
AVithin the sound-shadow of this screen a system of co-ordinates, in a 
plane parallel with the floor, was set up by means of light rods divided into 
centimetres. 
It had been shown by the author that rays of sound diverging from such a 
source, diffracted round the edges of the screen, on meeting after having 
passed through paths differing by half a wave-length, will neutralise each 
other and produce comparative silence. By moving an unison resonator 
along these co-ordinates the points of interference are easily detected, and 
are found to be situated as those of the diffraction of light. This was done 
in a closed room with still air. The doors and windows were then thrown 
open, and stove registers allowed to admit streams of air at nearly 100° C. 
The diffraction phenomena instantly disappeared. 
The theory thus corroborated was applied to the Baltimore Academy of 
Music, which is specially arranged to prevent the formation of air-currents of 
unequal density. The whole supply of fresh air is admitted at the back of 
the stage, warmed, and led across it to the proscenium diagonally towards 
the roof, passing into the ventilating tower over the great chandelier at the 
Tate of about 15,000 feet per minute. The room is singularly easy to speak 
and sing in. 
