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is the object of placing it under water, for if that precaution is not 
taken the tube will sometimes become very hot, and explode. The 
gutta percha covering is to prevent the spark passing through the 
water, and to oblige it to pass through the tube. It is sufficient, 
as I have said, to cover one wire. If a drop of water has been 
enclosed in the tube along with the chemical substance, the 
* 
colours of the spectra are displayed with more vivacity; hut if this 
is done, it is absolutely necessary to have the tube well under 
water. The bright light given off under these circumstances by 
strontia, sodium, thallium, and many other substances, is very 
beautiful, and so permanent that at the close of the experiment 
the original grain or half grain of the substance does not appear 
diminished, and even the drop of water is found remaining 
unchanged. Provided always that the chemical substance is one 
not liable to decomposition under these circumstances of heat and 
moisture. In these experiments a small Ruhinkorff’s coil was 
found to answer better than a very large one. 
This method might be usefully applied to the illumination of 
microscopic objects by homogeneous light. If the tube were 
placed immediately under the stage of the microscope, the full 
intensity of the yellow light would fall upon the object. 
All these experiments were made in the Physical Laboratory of 
the University of Edinburgh by the kind permission and assistance 
of Professor Tait. 
II. On the Nicol Prism. 
Many years ago, when this beautiful and useful optical instrument 
was new and very little known, I wrote a paper in a scientific journal 
calling attention to its merits, and recommending its use. It was 
first described by its inventor in Jameson’s Journal for 1828, p. 83. 
The title of the paper being “ On a Method of so far increasing the 
Divergency of the two Rays in Calcareous Spar that only one Image 
may he seen at a time I This paper was reviewed in Poggendorffs 
Annalen for 1833, p. 182, who says—That he perused Mr Nmol's 
account of his invention with very little hope of its proving suc¬ 
cessful, but that having constructed the instrument, he found that 
nothing could answer more perfectly than it did.' Having read 
this testimony to its merits, I had one made by a London optician, 
