246 PROCEEDINGS OF THE SOCIETY. 



" new invented doulile Microscope " was a wonderful piece of apjiaratns. 

 and he wondered how the ilUimination was managed. In the illustration 

 in " Carpenter," a caudle was placed close under the condensing lens, 

 which would lie either blacked or ci-acked, or both, in a short time. 



Mr. Gabb, in referring to the question of the advent of the substage 

 condenser, pointed out that it was in the John Marshall Microscope on 

 exhibition, and which dated from about 1690 — a scientific instrument 

 of wonderfully good performance for its time. 



Mr. Court considered that some of the early objectives by Powell, 

 especially the lower powers, dating as far back as 1841, were practically 

 as good as those in use to-day, apochromatics excepted. The reflectors 

 for the reflecting Microscopes were constructed after the form of the 

 Newtonian telescope ; there was a small aperture above the stage, and 

 the light from the object on the stage was reflected by a plane mirror, 

 fixed at an angle of 45°, ujjon a concave elliptic speculum fixed at the 

 further end, which was again reflected towards the eyepiece. The only 

 means of changing the power in this form of Amici's Microscope was to 

 change the power of the eyepiece ; several eyepieces of varying power 

 were supplied with that instrument. John Cuthbert, who made the 

 smaller I'eflecting Microscope, was especially celebrated for the short 

 focus, and the accuracy with which he ground the mirrors of his 

 reflecting telescopes ; these instruments were produced under the direc- 

 tion of Dr. Goring. 



Mr. Blood was convinced from the pattern of the mount that the 

 Powell objective was later than the stand, but could confirm what had 

 been said, that some of the lenses of about that date were of great 

 excellence. He endorsed what had been said already about the extreme 

 interest of that exhibit. He remarked upon the overpowering influence 

 of the chromatic aberration compared with the spherical aberration in 

 the better of these images, and inquired whether any devices were used 

 to get rid of the chromatic aberration, as, for instance, monochromatic 

 light, either by coloured glasses, or coloured solutions, or the sodium 

 flame. On looking at the images among the uncorrected Microscopes, 

 he noted how very soon the limit of useful magnifications was reached, 

 and when the power was higher the aberrations very soon become quite 

 distressing. 



Mr. J. E. Barnard then demonstrated by means of the ultra- 

 Microscope : — 



(1) The manner in which sulphur emerges from solution. In 

 explaining the experiment, Mr. Barnard reminded the Fellows that if 

 dilute sulphuric acid was added to a dilute solution of hyposulphite of 

 soda, a deposition of sulphur results. Theoretically it was assumed 

 that the sulphur at once began to separate out, but was at first in a 

 highly divided state, possibly molecular particles size ; these then 

 aggregated in increasing volume until they became visible sulphur 

 particles. He could not accept this theory, because if correct a 

 luminous light cone would be apparent where the substances are not in 

 complete solution before the particles became visible: in cases where 



