PROCEEDINGS OP THE SOCIETY. 
309 
the test was sufficiently difficult. On the whole, the result seemed to 
him quite satisfactory, but there was a certain muddiness in the 
picture, due to the use of an old amplifier which needed repolishing. 
He was having some new amplifiers made with which he expected to 
get more sparkling pictures.’’ 
The photograph accompanying the letter was handed round for 
examination ; it showed the striae with remarkable distinctness. 
Mr. Ingpen inquired if any question as to priority in regard to 
the invention or use of this prism arose out of the paper. He had 
himself seen a prism with four faces, made by Messrs. Powell and 
Lealand, from drawings by Dr. Edmunds, which appeared to more 
than cover the ground aimed at by the prisms now shown. 
Dr. Edmunds, in reply to a question from the President, said that, 
as to the prism to which Mr. Ingpen had referred, he had not thought 
it of sufficiently marked novelty to make it the subject of a formal 
communication, but he would be pleased to submit it to the next 
meeting. Upon the question of priority, it would be recollected that 
in 1856 Mr. Wenham described a small right-angled prism, which he 
attached to the under surface of the slide with oil or balsam, and used 
for oblique illumination. This application of a right-angled immer- 
sion prism was clearly due to Mr. Wenham. What was due to 
Colonel Woodward in his beautiful work was not the invention of the 
right-angled prism, but its combination with a pin-hole shutter, 
through which a small beam of parallel light could be thrown into a 
balsam-mounted object outside the angle of 41° from the optic axis ; 
thus demonstrating that pencils outside 82° balsam-angle could be 
used to form an image if only the objective were of sufficiently large 
aperture. With reference to his own prism, he had had the advantage 
of working with that sent over by Colonel Woodward, and finding it 
difficult to manage, and that its corners practically prevented its 
rotation under the slide, he had had another form constructed by 
Messrs. Powell and Lealand, and this had proved most useful and 
very easily managed. It was made from rather more than a hemi- 
sphere of glass, of which the spheroidal surface was ground down into 
four faces making practically two right-angled prisms at right angles 
to each other, one having its faces inclined to the plane surface at 
41° and 49°, and the other at 30° and 60°. The prism was set in a 
simple brass tube with a slot for each face, and fitting below into the 
substage. The faces were so arranged that an object in focus was 
illuminated equally through each face by light entering it at the 
normal. The upper surface was ^ inch in diameter and was made 
optically continuous with the slide by oil or glycerine. By simply 
turning the substage, the prism gave unrefracted light at 30°, 41°, 
49°, and 60° from the optic axis, and each angle could be varied a 
little without practical detriment. The light reflected from the top 
surface of the prism passed out through the other side at the normal, 
and thus glare was prevented. The prism worked charmingly. 
Mr. Crisp said that the interest of Colonel Woodward’s present 
apparatus arose entirely out of the angle of aperture discussion. 
Colonel Woodward certainly had never claimed and did not intend to 
