PROCEEDINGS OF SOCIETIES. 
209 
Mr. Slack said that much higher than that could be used ; he had 
himself employed powers as high as ¥ V inch to show the bands with 
a portion of a blood-corpuscle. 
The President said it was simply a question of light ; in making 
these observations it was merely necessary to make the object fill the 
entire field of view, so that the eye should not be distracted by the 
light coming in at the sides. He therefore used either a power suffi- 
cient to make the object fill the field, or an apparatus to stop out all 
light except that which came through the object. He should in all 
cases advise the use of the lowest possible power in order to obtain 
the largest possible amount of light, so that good definition might be 
obtained with the narrowest slit. 
Mr. Frank Crisp inquired what advantage the new eye-piece had 
over the old one. With all deference to the President, it appeared to 
him to be more complicated instead of more simple, for where one had 
to push in this and pull out that, it seemed to be more troublesome 
than exchanging the eye-pieces. 
The President said his chief object had been to obtain portability ; 
and by reducing the number of pieces of apparatus by merely adding 
a prism and a slit to an ordinary eye-piece, he thought he had con- 
trived a method by which the instrument was rendered as portable as 
possible. It should, however, be mentioned that the instrument with 
which he performed 90 per cent, of his work was the binocular form, 
and that he only resorted to the old form when the object was too 
small to be examined in any other manner. 
Mr. Slack believed that none of the cloudy bands were resolvable 
by higher powers into a series of minute bands, and this might be 
taken as an indication of an exceedingly complex combination of rays. 
The President said there were bands in a few substances which 
could be divided into two ; but these were quite exceptions to the 
general rule, which was that of a dark band shading off towards its 
edges. The great interest was of course centered in the connection 
between these bands and physical questions. 
Dr. Matthews inquired if the prism was in any way affected by the 
quality of the object-glass, or in the matter of over or under correction ? 
The President said it would not signify at all ; and even sup- 
posing that the object-glass were quite chromatic, it would not affect 
the result. It was, however, important to have a correction at the 
eye-piece in order to get the whole of the spectrum in focus at the 
same time. 
Dr. Pigott supposed it would be in some degree modified by the 
lenses which were placed between the eye-piece and the eye. 
The President said the plan was simply to make the eye-piece a 
rough achromatic combination. He had never very closely examined 
the construction of it, as he trusted to Mr. Browning in the matter ; 
but it was a double convex and a plano-concave in combination, and 
in the result it gave a beautifully flat field. The adjustment in the 
eye-piece was necessary, as otherwise if the focus were adjusted for red 
the blue end would be out of focus. By the plan which he had 
adopted lately, the same result was arrived at by a little plano-convex 
lens placed below the object. 
