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Transactions of the Society. 
last century to moderate the excessive amount of colour which lenses 
then exhibited. I refer to two patents granted to Dr. Robert Blair, 
Surgeon in the Royal Navy and afterwards Regius Professor of 
Astronomy in the University of Edinburgh. The first patent * is for 
“ A Method to Improve the Refracting Tellescope and other Optical 
Instruments,” Blair’s method being to fill the spaces between the 
lenses with fluid. He says, “ The present improvement consists in 
removing the impediments arising from the imperfection of flint glass 
to the construction of refracting telescopes with large apertures and 
high magnifying powers, in correcting the errors arising from a 
difference of refrangihility of the rays of light and those from the 
spherical figures of glasses more perfectly, and transmitting more 
light where the apertures are equal, than can be done in the common 
achromatic telescope, by rejecting the imperfect kind of glass called 
wffiite flint glass from the compound object glass, and in using in lieu 
of it a dispersive fluid. And in general the improvement consists in 
refracting the rays of light without dispersion of colour in dioptrical 
instruments, whether composed of lenses or prisms, where refraction 
without colour is wanted by a combination of crown glass, or an^ 
other kind of glass except flint-glass, with a fluid of different dis- 
persive power from that of the glass with which it is combined, and 
that either by opposite refractions as at present practised with different 
kinds of glass, or by a single refraction in the manner above explained. 
By accurate experiments most metallic solutions were found to possess 
this dispersive power ; or solution of corrosive sublimate mercury, 
either alone in spirit of wine, or in water with the addition of crude 
sal ammoniacum, disperses in a very considerable degree, hut is 
greatly inferior in this respect to a chemical preparation called the 
caustic or butter of antimony,! which, in its strongest state, dispersives 
three times more than crown glass. Some essential oils were also 
found to possess this property in a sufficient degree to he useful for 
optical purposes. Those who are conversant in optics will not be at 
a loss in adapting the curvature of the lenses to the dispersive power 
of the fluid whose properties they have examined, so as to produce a 
colourless refraction, and will also perceive that, from having a choice 
of so great a variety of mediums of different refractive and dispersive 
powers, the errors of homogeneal rays arising from the spherical 
figure of the lenses may be more accurately corrected than when 
two kinds of glass of nearly the same density are made use of ; this 
subject having been treated of long since by opticians of the greatest 
eminence. And it is well known that an accurate union of rays of 
all sorts, by the object glasses of telescopes and microscopes, is the 
chief thing requisite to bring these instruments to the greatest 
degree of perfection of which they are capable.” 
Blair continued his researches, and in 1791 took out a second 
* No. 1473, dated 26th April, 21st and 24th May, 1785. Reprinted in 1856. 
f Chloride of antimony. 
