1^4 Dr Brewster on a Monochromatic Lamp 
some time, a portion of unevaporated water, mixed with a small 
quantity of alcohol, will remain at the bottom of the dish, in a 
«tate unfit for combustion. This water may be taken up by a 
sponge, or it might be prevented from accumulating, by having 
a fountain of pure alcohol, from which the exhausted strength 
of the diluted fluid could be renewed. 
The monochromatic lamp being thus completed, I lost no 
time in applying it to the illumination of microscopic objects. 
The effect which it produced far exceeded my expectations. 
The images of the most minute vegetable structures were precise 
and distinct, and the vision in every respect more perfect than it 
could have been, had all the lenses of the microscope been made 
completely achromatic by the most skilful artist. 
Independent of its use in microscopical observations, the 
monochromatic lamp will find an extensive application in various 
branches of the arts and sciences. In certain cases of imperfect 
vision, where a number of coloured images are formed by the 
separation of the fibres of the crystalline lens, a homogeneous 
light will improve the vision, by removing the prismatic tints, 
which obliterate the principal image. In illuminating the wires 
of transit instruments and micrometers; — in graduating the 
limbs of divided instruments, which is generally done by candle- 
light ; — in reading off* the same divisions in fixed observations ; 
— in forming signals in trigonometrical surveys ; — in obtaining 
correct and uniform measures of refractive powers ; — -in measur- 
ing the separation of the two pencils in doubly-refracting crys- 
tals ; — in determining the focal lengths of lenses ; — in observing 
various optical phenomena, where the light is decomposed; — 
in these, and, in general, in all delicate works, where correct 
vision is essential, the employment of a homogeneous flame will 
be found to confer the most signal benefits. 
Explanation of Fig. 5, 6. of Plate II. 
Fig. 6. Represents one form of the monochromatic lamp, 
where A is the reservoir containing the diluted alcohol, 
which descends by the channel ABCD to the broad wick 
E, which i^ generally made of sponge. A frame of wire- 
gauze F moves round a hinge H, so that it can be brought 
over the flame, and made to descend, when hot, upon the 
