50 Proceedings of the Royal Society 
heat, which in one or two cases has already given interesting 
results. 
We propose also to study the action of phosphide of sodium on 
other haloid derivatives — especially on chloride of phenyl. 
6. Notice of an Easy Method for determining the Position of 
the Principal Focus of an Object-Glass. By Edward Sang. 
Just as the past progress of astronomy has, in a great measure, 
been due to the improved construction of our instruments, so its 
future progress must depend much on the accurate confection of 
our telescopes, and most of all on the excellence of the object- 
glasses. 
In the actual formation of a compound lens, after the thicknesses 
and curvatures have been fixed, we have to make the tools to the 
proper shapes and with those to grind and polish the glass. It is 
impossible, even with the greatest care, to bring the workmanship 
so close to the computations as that appreciable differences may 
not be found, and hence the finished lens hardly ever comes so 
near to our expectations as that it may not be neeessary to test it 
by actual trial. One important matter to be so settled is the 
exact position of the principal focus, since that is needed for 
determining the length of the tube. 
The obvious course of procedure is to place the lens in a 
temporary frame and to direct it to the sun, to a star, or to some 
very remote terrestrial object. This, however, requires clear air 
and open space such as is not always to be had in a large town. 
To bring the whole operation within the limits of the workshop I 
have had recourse to the following expedient : — 
The lens, in its setting, is secured against the surface of a flat 
mirror or speculum, and is set up on a table of sufficient length. 
A well illuminated object is then placed approximately in the 
focal plane. The light from this object, after having passed through 
the lens is reflected from the mirror and again passes through the 
lens, being converged to form an inverted image. 
If the object be placed within the focal distance, the image will 
