At six P.M. this day I reached the Montanvert; and the same evening, wrapping my plaid around me, I wandered up towards Charmoz, and from its heights observed, as they had been observed fifteen years previously by Professor Forbes, the _dirt-bands_ of the Mer de Glace. They were different from any I had previously seen, and I felt a strong desire to trace them to their origin. Content, however, with the performance of the day, and feeling healthily tired by it, I lay down upon the bilberry bushes and fell asleep. It was dark when I awoke, and I experienced some difficulty and risk in getting down from the petty eminence referred to. The illumination of the glacier, as remarked by Professor Forbes, has great influence upon the appearance of the bands; they are best seen in a subdued light, and I think for the following reasons:-- The dirt-bands are seen simply because they send less light to the eye than the cleaner portions of the glacier which lie between them; two surfaces, differently illuminated, are presented to the eye, and it is found that this difference is more observable when the light is that of evening than when it is that of noon. It is only within certain limits that the eye is able to perceive differences of intensity in different lights; beyond a certain intensity, if I may use the expression, light ceases to be light, and becomes mere pain. The naked eye can detect no difference in brightness between the electric light and the lime light, although, when we come to strict measurement, the former may possess many times the intensity of the latter. It follows from this that we might reduce the ordinary electric light to a fraction of its intensity, without any perceptible change of brightness to the naked eye which looks at it. But if we reduce the lime light in the same proportion the effect would be very different. This light lies much nearer to the limit at which the eye can appreciate differences of brightness, and its reduction might bring it quite within this limit, and make it sensibly dimmer than before. Hence we see that when two sources of intense light are presented to the eye, by reducing both the lights in the same proportion, the _difference_ between them may become more perceptible. [Sidenote: BANDS SEEN BEST BY TWILIGHT. 1857.] Now the dirt-bands and the spaces between them resemble, in some measure, the two lights above mentioned. By the full glare of noon both are so strongly illuminated that the difference which the eye perceives is very small; as the evening advances the light of both is lowered in the same proportion, but the differential effect upon the eye is thereby augmented, and the bands are consequently more clearly seen. (7.)