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VIII. — A New Illuminating Apparatus. 
By E. M. Nelson, F.R.M.S. 
( Read 20 th May , 1891.) 
In the direction of monochromatic light very little has been done. 
This may be attributed to the unsitisfactory results already obtained. 
Who, for instance, has seen a critical image with monochromatic light ? 
Many have witnessed attempts, and some of us have ourselves tried 
experiments. There has been one universal verdict to all these 
efforts — viz. that of dismal failure. The experiments have taken two 
forms: first, that of obtaining monochromatic blue light by inter- 
posing absorption media, the other by prism dispersion. 
The first may be dismissed in a word : so far as is known there 
does not exist a medium that will only pass a blue ray. Ammonio- 
sulphate of copper passes any amount of red light, blue glass does the 
same. The examination with a spectroscope of the light which has 
passed through any of these absorbing media immediately dispels the 
idea that it is monochromatic. Before proceeding I may say that the 
best results in this direction were obtained by using two thicknesses 
of pot cobalt glass supplied by Messrs. Powell and Lealand. The 
spectrum through these curiously agrees with that through Rainey’s 
light-modifier, which was composed of three different tints of glass ; an 
account of which will be found in the Transactions.* For monochro- 
matic light in the strict sense we are thrown back on prism dispersion. 
Instruments on this plan have been made, notably by Zeiss on a 
Hartnack model. The Zeiss monochromatic apparatus is probably 
very suitable for obtaining a spectrum, but in its design the require- 
ments of the Microscope are absolutely ignored. It consists of a slit ; 
a low-angled achromatic lens having the slit at its principal focus ; 
two prisms ; and another low-angled achromatic lens very similar to 
the first. The rays diverging from the slit are parallelized by the 
first lens, and after passing through the prisms are received by the 
second lens and by it brought to a focus. With this apparatus the use 
of a substage condenser is impossible because it yields convergent rays. 
It must be borne in mind that none of Abbe’s condensers will 
focus parallel rays ; much less therefore will they focus convergent. 
If it is used, as intended, without any additional apparatus the cone 
from the low-angled condensing lens is too narrow to be of any 
service. As therefore this apparatus will work neither with nor 
without a condenser, it becomes mere lumber in the microscopist’s 
cabinet. I never heard of one that had been used after its first trial. 
The apparatus I am exhibiting this evening is, as may be seen, only a 
makeshift ; but I claim that it has for its end the requirements of the 
modern Microscope. 
* Trans. Micr. Soc. London, N.S. ii. (1854) pp. 23-4. 
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