TRANSACTIONS OF THE SECTIONS. 5 
results were obtained by the correction of his error, how superior must the micro» 
scope be in which the concentric adjustment of its lenses is effected! While 
opticians, indeed, confine themselves to the use of only two kinds of glass, of 
different refractive and dispersive powers, we can hardly expect much improvement 
in the microscope, unless by the substitution of achromatic lenses in the eye-piece, 
and by an infallible method of centring each lens, and each group of lenses, in the 
instrument. The successful application of two pairs of adjusting screws to each of 
six lenses, and also to those of the cye-piece, may be a difficult task, but it is not 
beyond the powers of mechanism. It is very obvious that Dr. Wollaston’s method 
of examining the centring of a triple object-glass is wholly inapplicable to the 
object-glass of a microscope. In submitting to examination an object-glass made 
by a distinguished optician, it was necessary to use a microscopic picture of the sun, 
and to examine the position of its images as reflected from the various surfaces of 
the lenses by means of a microscope, the object-glass of which was brought in | 
contact with the outer lens of the object-glass to be examined. By separating the 
two object-glasses, I observed in succession a series of twenty-four images appearing 
and disappearing in succession. These images occupied different parts of the field, 
and I could not succeed by the most careful adjustment of the apparatus employed 
in placing them in the same axis. These images had various sizes, and were in 
various states of colour, some highly coloured, and some purely white. They had 
also various sizes, many with fine planetary discs, of different magnitudes; some 
like the smallest fixed stars which it was difficult to descry, and almost all of them 
exhibiting the most beautiful concentric diffracted rings when put out of focus. 
Two or three images often appeared in the same part of the field, in immediate suc- 
cession, while similar pairs arose at a distance from each other. Although [ often 
succeeded in uniting two or more of these images, yet the effect of this was to place 
others at a greater distance; and I had no hesitation in coming to the conclusion, 
that the lenses of the object-glass which produced these images were imperfectly 
centred. Having had occasion to see at the Paris Exposition, and more recently 
at Florence, the superior performance of Prof. Amici’s microscopes, I cannot omit 
the present opportunity of urging philosophers and opticians, as I have often done, 
te correct the colours of the secondary spectrum by fluids or solids of different 
dispersive powers. Prof. Amici has donethis. In his object-glasses, Nos. 1 and 2, 
of low powers, he employs five different refractive and dispersive substances. In 
his powers Nos. 3, 4, and 5, he employs five such substances ; and in his highest 
power, No. 6, he employs six. In recommending, as I have often had occasion to 
do, the employment of diamond and other gems in the construction of compound as 
well as simple microscopes, I have been met with the objection that they are too 
expensive for such a purpose, and they certainly are for instruments intended merely 
to instruct and amuse; but if we desire to make great discoveries, to unfold secrets 
yet hid in the cells of plants and animals, we must not grudge even a few diamonds to 
reveal them. If Mr. Cooper and Sir James South have given a couple of thousand 
pounds for a refracting telescope, in order to study what have been miscalled 
“dots” and “lumps” of light on the sky; and if Lord Rosse has expended far 
greater sums on a reflecting telescope for analysing what have been called “sparks 
of mud and vapour” encumbering the azure purity of the heavens, why should not 
other philosophers open their purse, if they have one, and other noblemen sacrifice 
some of their household jewels to resolve the microscopic structures of our own 
real world ;—to unravel mysteries most interesting to man; and disclose secrets 
which the Almighty must have intended that we should know? 
On a new Polarizer, resulting from a Modification of the Prism of Nicol. 
By M. Leon Foucautt, Paris. 
When it is proposed to polarize in a complete manner a pencil of white light, the 
best means is to recur to the prism of Nicol; but if a pencil of a certain volume is 
; 
Bi 
to be acted on,—from four to five centimetres diameter, for example,—Nicol’s prism 
becomes expensive and difficult to realize, on account of the scarcity of the beautiful 
specimens of the spar of Iceland. The cut adopted for the construction of the 
prism of Nicol entails necessarily a great cost of material, To have the prism entire, 
