734 
Notes . 
Air 
equal 
I'OO 
Water 
i*33 
Diatomaceous silex | 
Sulphuric acid ) 
>» 
1 ‘43 
Canada balsam 
i-54 
Bisulphide carbon 
Solution of sulphur in bisulphide car- 
ff 
i-68 
bon (approximately) 
>> 
175 
Sulphur > • •• . ••• ••• 
Solution of phosphorus in bisulphide 
>> 
2*11 
carbon (approximately) 
>> 
2*10 
Taking the difference between the refractive indices of Diatom- 
aceous silex and the several mounting media, the following 
results are obtained : 
Water 10 
Canada balsam 11 
Bisulphide carbon ....... 2 5 
Solution of sulphur in bisulphide carbon ... 32 
Solution of phosphorus in bisulphide carbon 67 
The practical result of the investigation appears to be— 
That it is essential, if the whole aperture of an objeaive is to 
be utilised, to mount minute structures in some medium 
other than air. 
That although the full aperture and revolving power are 
secured by mounting in balsam, it gives nevertheless nearly 
the faintest image of all. 
That a solution of phosphorus is, as far as visibility is con- 
cerned, by far the most effeaive, but the difficulties attending 
its use must render it unpopular. 
The next best is a solution of sulphur in bisulphide of carbon 
(although pure bisulphide is very good), and with these there is 
no technical difficulty whatever. This medium can easily be 
secured by using the solution employed by Mr. Browning in 
making his bisulphide prisms. A ring being made on the slide, 
and a drop of the sulphur solution or pure bisulphide being placed 
in its centre, nothing is necessary but to place over it the thin 
cover with its adhering diatoms, press it down on the still moist 
ring, running round it a somewhat copious margin of the cement. 
When dry, to protea it from the water of the ordinary immersion 
lenses, it is desirable to give it a coat of gold-size or shellac 
varnish. 
