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VIII . — On some Organic Substances of High Refractivity, available 
for Mounting Specimens for Examination under the 
Microscope. 
By H. G-. Mad an, M.A., F.C.S., Fellow of Queen’s College, 
Oxford. 
( Read 20 tli April , 189S.)] 
In the course of experiments made with a view of obtaining some 
cement having the same index of refraction as Iceland spar for the 
ordinary ray (viz. 1*66), and therefore suitable for use in certain 
forms of polarising prism made of that material, I have had occasion 
to prepare, or procure, and examine several of the less known organic 
compounds of high refractivity. One or two of these do not seem 
as yet to have attracted the notice of microscopists, and I am induced 
to bring them before the Society, in the hope that they may be con- 
sidered worth a trial by some who have had more experience than 
myself in the mounting and critical examination of specimens. 
I must say at once that I have not yet been successful in finding 
an entirely satisfactory material ; but I have thought it worth while 
to give a short account of what I have done up to the present time, 
in the hope of stimulating others in the search and of receiving 
suggestions for future experiments. 
In the first place, let me state clearly and definitely what we 
want ; the requirements of physicists and microscopists being in 
this matter pretty nearly identical. We want an irreproachably 
colloidal substance, with no tendency whatever to crystallise under 
any conditions, colourless, transparent, solid and tough at ordinary 
temperatures, but easily liquefied (or at any rate rendered viscous) by 
a gentle heat, stable under ordinary conditions of light and tempera- 
ture, with no tendency to act upon or injure even delicate preparations, 
and — most important point of all — possessing a very high refractive 
power on light. 
Perhaps we may consider Canada balsam (and one or two other 
similar gum resins) to have all the properties referred to except the 
last ; and we may state our aim to be the discovery of a balsam-like 
substance, with a refractive index for rays of mean wave-length of 
not less than 1*66. 
I do not propose to consider now any of the compounds of metals, 
such as arsenic tribromide, thallium chloride, mercury-potassium 
iodide, &c., which have been tried. Such substances, though useful 
for special purposes, and fulfilling excellently the requirement of high 
refractivity, have not the permanency and harmlessness which are 
desirable ; they would quickly ruin, for instance, a polished surface 
of Iceland spar. I shall only draw attention to a few organic bodies 
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