In making an examination with the Holmgren test, it is almost unnecessary that the candidate should take up a skein out of the heap of wools to form a preliminary diagnosis. The colour blind will not at once pick out an evident match, but will hesitate and evince a desire to appear very accurate in his choice. This indicates at once that there is something amiss. He probably will pick up a skein of the right colour, place it against the test-skein, lay it down and again take it up. Or he will pick up a skein which is evidently incorrect and do the same thing, but perhaps he will return it to the heap and take up another which is equally bad. He will fumble over making his matches, and eventually have a heap by him which will at once tell the examiner that he is colour defective. I may as well give you an idea of the colours which the colour blind will pick out by a simple experiment. The heap of wools is on the table, and in the pure white of the electric arc light, which is thrown on it from the lantern, every colour is distinct in hue and in intensity. On one side are placed the two important test-skeins, the pale green and the pink. There can be no doubt but that in that heap of wools there are a large number which can be matched with each of them. The red-blind, be it recollected, sees no red, and if I can place in front of the lens of the lantern some medium which cuts off the red as completely as possible, the audience as well as myself will see the colours approximately as the red-blind would do. Such a medium is found in the same blue-green glass that is used for signals on most railways and on board ship. The green-blind, on the other hand, see no green, and if a medium can be found which when placed in the path of the light allows no green to pass, the colours in the heap being deprived of the green would be such as would very nearly be the same as this type of colour blind would see. This glass is covered with a film of collodion in which fuschin and blue have been dissolved. It transmits a fine purple and should answer our purpose. That these two media are what we require can be readily demonstrated by placing them in front of the slit of the collimator of our colour apparatus and throwing the spectrum on the screen. The spectrum of white light is now on the screen, and when we place the blue-green glass in front of the slit, we see that the red is very nearly entirely extinguished, whilst if we substitute for it the dyed collodionized glass the green is absent. Now, placing the first glass in front of the lantern lens and switching on the current, the wools are illuminated with the bluish-green light. The green test-skein appears green, and we can proceed to make our matches, picking out the colours which appear the same, but taking no heed as to their lightness or darkness. A dozen skeins are now picked out, and I think the audience will agree with me that the matches as viewed in the green light are accurate.