104 Dr Brewster on Vision through Coloui'ed Glasses^ 
geneous light, without obscuring the object, we niust seek foi 
that colour which produces the shortest spectrum, with the 
greatest illumination. In this examination, I have tried a great 
variety of coloured glasses, and have found, that a yellozvish- 
green glass has the property required. It almost entirely ab- 
sorbs the extreme red rays, and extinguishes a very great pro- 
portion of the blue extremity of the spectrum. Hence, it not 
only relieves the eye, by attenuating the incident light, but it 
improves the image, by diminishing the error arising from its 
different refrangibility. 
Having thus considered the influence of coloured media upon 
simple vision, it becomes interesting to inquire how far the te- 
lescope and tnicroscope are susceptible of improvement, by the 
use of coloured lenses. As the objects to which the telescope iS 
applied, do not admit of artificial illumination, the absorption of 
the obnoxious rays can only be resorted to, when there is a con- 
siderable intensity of light. In viewing the spots of the Sun, 
for example, and in examining Venus and Jupiter, when near 
the Earth, some benefit may be derived from the interposition of 
coloured lenses ; but it is principally by extinguishing the se- 
condary tints which remain, even in the best achromatic tele- 
scopes, that we anticipate any decided advantage. 
With the microscope, however, the case is quite different. 
The power which we possess of illuminating artificially the ob- 
jects under examination^ enables us to compensate the loss of 
light by absorption, and as we have also the apertures of the 
lenses under our controul, we may avail ourselves to a very great 
extent of the application of coloured media. 
In the construction of single microscopes, I have derived 
great advantage from usiiig both red and green lenses; particu- 
larly when the outline or form of an object was required. In 
compound microscopes, the lenses may be made either of the 
same or of different colours, or only one of the glasses may be 
coloured ; and the kind of light to be absorbed^ may be regu- 
lated by the colour of the object under examination. 
In order, however, to derive from coloured glasses the full be- 
nefit which they are calculated to afford, the compound micro- 
scope should be constructed on a scale of unusual magnitude. 
I had occasion many years ago to point out the advantages of 
