496 
PROCEEDINGS OP THE SOCIETY. 
prisms ; this was used with a side reflector — the President would per- 
haps remember it.* 
Mr. Swift said he had never seen one of this kind, and did not know 
of any which carried out the idea in the way Mr. Ives had done it. 
The President thought the method was a very valuable one, and he 
did not remember ever having seen this idea carried out in this way 
before. It certainly seemed preferable to the method of a thick film of 
silver with a hole in it. 
Mr. Swift said there certainly was some difficulty in centering the 
eye properly in the old form which did not exist in the one before them ; 
the pencil used for making the drawing being seen with a considerable 
amount of ease whilst tracing over the object under observation. 
In reply to an inquiry, if this really gave monochromatic or only 
coloured light, and what was the wave-length, — 
Mr. Ives said that, of course, the light was not strictly monochro- 
matic, although it was all spectrum green. It was a mixture of the 
pure green in the spectrum at the E line, with some yellow-green on 
one side and blue-green on the other. The screen transmitted very 
freely a well defined band in the spectrum, with the E line in the 
middle of it. A screen could be made to transmit a broader or narrower 
band of the spectrum, and more towards the red or the blue, if desired. 
The thanks of the meeting were unanimously voted to Mr. Ives for 
his exhibits. 
The President exhibited and described two old Microscopes, one of 
which, made by Benjamin Martin, probably dated from about 1770, and 
the other, made by Cary, somewhere about 1740 (see p. 473). 
The President said that Mr. B. W. Priest had taken the trouble to 
bring down to the meeting a remarkably beautiful collection of micro- 
scopic preparations of Sponges for exhibition. He hoped Mr. Priest 
would favour them with a few remarks upon the subject. 
Mr. Priest said that he had much pleasure in responding to a request 
from the Society that he would give an exhibition of slides of sponges, 
but he had done so upon the understanding that it would be impossible 
for him to read a paper describing the different specimens exhibited, as 
he felt he could not, in the time at his disposal, do justice to so vast a 
subject. He had brought a selection, which he trusted would be suffi- 
ciently characteristic, of the order Calcarea and the three sub-orders of 
Silicea, namely, the Monaxonidae, Tetractinellidae, and Hexactinellidae. 
He had placed cards with each instrument specifying the name of the 
exhibit, and might perhaps call attention more particularly to the 
Hexactinellidae, on account of their extreme beauty and complexity of 
detail. There were also some slides of freshwater sponges, showing the 
statoblasts, so named by his late friend Mr. H. T. Carter of Budleigh 
Salterton, because of their strong resemblance to the statoblasts of the 
* Since the meeting Mr. Beck has written to say that the camera referred to was 
made by Nachet, and had a gilding used instead of silvering. 
