SCIENTIFIC SUMMARY. 
443 
globule in originating combinations. To the optician it is as needful as the 
callipers and straight-edge to the engine-litter — every glass is separately 
tested by it. Its familiar readings show whether the work is going on 
right or wrong ; by the indications of inward or outward coma whether the 
oblique pencils are correct, and finally, the least chromatic or spherical error 
can be ascertained by its means. It may be 1 well-known to mathematicians 
that these globules are not perfectly spherical ’ (and mathematicians will 
be correct in all things), but setting aside the fact that the more minute the 
particle the nearer it approaches to a true sphere, it happens that shape is 
not of the smallest consequence, or whether it is illuminated by oblique 
light, for it is not the globule but the absolute point of light reflected from it 
that is used. The diameter of a mercury globule for correcting the highest 
powers from a \ upwards is only the one five-thousandth part of an inch. 
Perhaps some one who thinks it may advance the subject, will be good 
enough to calculate the size of the image of a small lamp-flame set at 4 
inches distance, reflected from the surface of a convex mirror of jo,§oo au 
inch radius. Dr. Pigott, by converting the microscope object-glass into a 
species of telescope, and viewing distant and minute discs of light, professes 
by means of the 1 Aplanatic Searcher ’ ! ! ! to have discovered spherical error 
in all our best glasses, to the existence of which everyone else has hitherto 
been blind. Doubtless a very imposing or ‘ striking ’ demonstration may be 
made out of this, but it is easy to demonstrate that by so doing we are sett- 
ing at naught the very qualities and advantages of large angular pencils. 
The conjugate foci are now so far distant, that large angular aperture no 
longer exists. A difference in the adjusting collar that would produce an 
enormous amount of spherical aberration, when the object-glass is tried on 
the globule test, or in its legitimate use as a microscope lens, is scarcely 
perceptible in the telescope arrangement, and though a badly-corrected glass 
may not form an image, yet I have no hesitation in affirming that a lens 
may be made to give perfect definition under the latter condition, that will 
prove utterly worthless as an objective for the microscope.” — Monthly Micro- 
scopical Journal , July. 
PHYSICS. 
The Construction of Thermopiles. — In a recent number of the Transac- 
tions of the Royal Society Lord Posse gives a paper on the construction of 
thermopiles, which, though we cannot abstract at any very great length, 
is of considerable importance. Alluding to his experiments, Lord Posse 
says that, although the above experiments are far less complete than he 
could have wished, they are sufficient to show that the sensibility of ther- 
mopiles may be considerably increased by diminution of the section of the 
bars composing them ; whether they may be with advantage reduced to a 
greater extent than he has already done he cannot say, but he is inclined to 
think that they may. He has ascertained from Messrs. Elliott that the 
alloys used by them in the construction of thermopiles, at the time when he 
received his from them, were 32 parts of bismuth + 1 part of antimony, and 
141 of bismuth + 1 part of tin. If allowance be made for the substitution 
