122 Dr Bi'ewster on a Monochromatic Lamp 
the spectrum that was best fitted for giving perfect vision, yet 
the quantity of light extinguished before the insulation of the 
extreme red ray was affected, was so great as to render the de- 
termination of little practical utility, excepting in cases where the 
outline of an object was to be observed. Had it been possible 
to insulate the most luminous rays of the spectrum as perfectly 
as the extreme red ones, the advantage would have been of very 
considerable amount ; but I have found this quite impracticable, 
and I venture to say, that the separation of homogeneous green 
or homogeneous yellow light, of any considerable intensity, can- 
not be effected by any coloured media with which we are at pre- 
sent acquainted. 
Abandoning, thei’efore, all hopes of obtaining from coloured 
media any farther improvement upon the microscope than what 
I had formerly announced, it occurred to me, that the object 
which I had in view might be obtained, if I could procure, from 
the combustion of inflammable substances, a homogeneous fame 
for illuminating microscopic objects. 
It had long been known, that a great quantity of homoge- 
neous yellow light was created by placing salt or nitre in the 
white flame of a candle, or in the blue and white flame of burn- 
ing alcohol A light, however, generated in this manner, was 
more fitted for a casual experiment, than for a permanent source 
of illumination ; and as insalubrious vapours are disengaged du- 
ring the combustion of these salts, I did not avail myself of this 
method of obtaining yellow light. 
After numerous experiments, attended with much trouble and 
disappointment, I found that almost all bodies in which the com- 
bustion was imperfect^ such as paper ^ linen, cotton, ^c. gave a 
light in which the homogeneous yellow rays predominated ; — 
that the quantity of yellow light increased with the humidity of 
these bodies ; — and that a great proportion of the same light was 
generated, when various flamies were urged mechanically by a 
blowpipe or a pair of bellows. 
* Edinburgh Physical and Literary Essays, vol. ii. p. 34, ; and Dr Thomas 
Young’s Nat. Phil. vol. i. p, 438. Mr Herschel informs me, that sulphur in a 
^lertain stage of its combustion produces a homogeneous yellow light. 
