Patents connected with the Microscope. By W. II. Brown. 27 1 
patent * for improvements in the same subject. It is described 
as “ A Method of Improving the Refracting Telescope and other 
Dioptrical Instruments.” Blair finds that the defects of object-glasses 
were owing to the fact that “ flint glass refracts the green light con- 
siderably less than crown glass, in proportion to the whole refraction 
of red and violet light, so that when the divergency of the red and 
violet light, caused by the refraction of the two mediums, is equal, 
the divergency of the red and green light is always greater in the 
crown glass than in the flint glass, and the divergency of the violet 
and green light is always less in the crown glass than in the flint 
glass. Those who are conversant in optical studies will perceive 
from this, that it is impossible to unite all the rays by any combina- 
tion of these two mediums ; for the correction of colour is most perfect 
when the red and violet light is united, and when this is effected the 
green light will always be refracted more than this united red and 
violet light.” 
After several experiments, Blair found that “ The marine and 
nitrous acids, which are dispersive fluids of considerable strength, 
instead of refracting the green light less than crown glass in propor- 
tion to the whole refraction of the red and violet light, were found to 
refract the green light more. ... I therefore mixed these two kinds 
of dispersive mediums, and thus obtained a medium which disperses 
the rays much more than crown glass, and yet causes all of them to 
diverge accurately in the same proportion in which they are made to 
diverge by the refraction of crown glass, which is the desideratum 
required to move entirely the aberration from the unequal refrangi- 
bility of light.” 
After cautioning opticians against using the medium in question 
without the necessary permission, Blair says, “ To dioptrical instru- 
ments constructed upon this principle I apply the term aplanatic, 
which denotes the absence of aberration, in order to distinguish them 
from those which have with impropriety been stiled achromatic. The 
dispersive medium which 1 have found to answer best is a solution of 
•antimony or mercury in the marine acid, but a variety of others may 
be used for the same purpose, and it is possible to remove the colour 
by a combination of two essential oils with glass, but in a more com- 
plex and less effectual way, than by using one dispersive medium, as 
above stated. It is well known that besides the aberration from 
unequal refrangibility, it is necessary to correct also the aberration 
from the spherical figures of lenses, but this has no connection with 
the Invention above specified. The colour may be entirely removed 
by using only one concave lens, formed of a dispersive medium; 
combined with one or with two convex lenses formed of an indispersive 
medium ; and if the dispersive medium be more dense than the in- 
dispersive medium, the spherical aberration may be also corrected 
without any addition ; but if the dispersive medium made use of be 
* No. 1800, dated 4th, 27th and 29th April, 1791. Reprinted in 1856. 
