602 Captain W. de W. Abney [May 6, 



The sun will be near rising, the white peaks beyond will appear 

 tipped with rose ; every colour will now be distinguished, though 

 they would still bo dull ; and, finally, the daylight will come of its 

 usual character, and the cold grey will give place to warmth of hue. 

 But there may be others who have never experienced this early 

 rising, and prefer the comfort of an ordinary English tramp to that 

 just described ; but even then they may have felt something of the 

 kind. In the soft autumn evening, when the sun has set, they may 

 have wandered into the garden and noticed that flowers which in the 

 daytime appear of gorgeous colourings — perhaps a mixture of red 

 and blue— in the gloaming will be very difieient in aspect. The 

 red flowers will appear dull and black ; a red geranium, for instance, 

 in very dull light, being a sable black, whilst the blue flowers will 

 appear whitish-grey, and the brightest pale yellow flowers of the 

 same tint ; the grass will be grey, and the green of the trees the same 

 nondescript colour. A similar kind of colouring will also be visible 

 in moonlight when daylight has entirely disappeared, though tbe sky 

 will have a transparent dark blue look about it, approaching to green. 

 These sensations, or rather lack of sensations of light and colour, 

 which as a rule attract very little attention, as they are common ones, 

 are the subjects of my discourse to-night. 



Experiments which can be shown to a large audience on this 

 subject are naturally rather few in number, but i will try and show 

 you one or two. 



We are often told that the different stages of heat to which a body 

 can be raised are black, red, yellow, and white heat, but I wish to 

 show you that there is an intermediate stage between black and red 

 heat, viz. a grey heat. An incandescence lamp surrourded by a tissue 

 paper shade, has a current flowing through it, and in this absolutely 

 dark room nothing is seen, for it is black hot. An increase of the 

 current, however, shows the shade of a dim grey, whilst a further 

 increase shows it as illuminated by a red, and then a yellow light. 

 A bunch of flow^ers placed in the beam of the electric light shows 

 every ccdour in perfection ; the light is gradually dimmed down, 

 and the reds disappt ar, whilst the blue colours remain and the green 

 leaves become dark. These two experiments show that there is a 

 colour, if grey may be called a colour, with which we have to reckon. 

 Now the question arises whether we can by any means ascertain 

 at what stage a colour becomes of this grey hue, and at what stage 

 of illumination the impression of mtre light also disappears, and 

 whether in any case the two disappear simultaneously. 



As all colours in nature are mixed colours, it is at the outset 

 useless to experiment with them in order to arrive at any definite 

 conclusion, hence we are forced— and the forcing in this direction 

 to the experimentalist is a very agreeable process— we are forced to 

 come to the spectrum for information. 



The apparatus on this table is one which I have before described 

 in this theatre, and it is needless for me to describe it again. I can 



