32 Frauenhofer on the Reactive and Dispersive Powers 
have the greatest brightness, will produce in the ratio of their 
intensity a worse effect than the violet ones, if the aberration 
for the latter is of the same magnitude. Hence, we must know 
tlie intensity of each colour in the spectrum, or in what ratio the 
impression of any colour of the spectrum is stronger or weaker than 
that of another colour. In order to measure this intensity, I 
constructed the following apparatus. 
To an eye-glass constructed for that purpose for the telescope 
of the theodolite, I applied a small plain metallic mirror, the 
edge of which being well defined, cut the field of the telescope 
in the middle, as shewn at a in Plate II. Fig. 2. It was placed 
before the eye-glass E, at an angle of 45°, and at the place of the 
image formed by the object-glass A. The eye-glass E is pulled 
out till the edge of the mirror, which ought to be vertical, is. 
distinctly seen. At the side of the eye-glass, and in a direction 
perpendicular to the edge of the mirror a, and to the axis of the 
telescope AE, I fixed a tube cB, cut in the direction of its 
length at h ; and in this cut I placed a narrower and a shorter 
tube MN, Fig. 1., which crossed the larger tube c B perj^ndi- 
cularly. In this narrow tube was a small flame, in the axis of 
the larger tube, which was supplied with oil from an external 
vessel. The narrow vertical tube b Fig. 2., or MN Fig. 1., 
had in the axis of the larger tube a small round aperture, turned 
towards the mirror a, by which the light of the flame fell upon 
it. By this contrivance, we perceive, in half of the field, th^ 
mirror a illuminated by the flame, and in the other half, one of 
the colours of the spectrum formed by a prism placed before the 
object-glass A. The nearer the tube b is brought to a, the more 
will the flame illuminate the mirror, and consequently we can 
obtain, at the same time, an impression produced on the eye by 
the light of the mirror (as seen by the eye-glass), of the same in- 
tensity as that which is produced by a colour of the spectrum 
in the other half of the field. The squares of the distances of 
the flame from the mirror for the different colours of the spec- 
trum, are then inversely as the ratios of their intensity. Though 
at first it appears difficult to compare the light of two different 
colours, yet it becomes easy by a little practice. The intensity 
of the light of the mirror approaches more to that of any colour 
in the spectrum, if at the same position of the eye-glass its ver- 
tical margin is less distinct. If the mirror is adjacent to a part 
