In a Paper on Vision through coloured Glasses, which I had lately the honour of submitting to the Society, I pointed out the advantages of coloured media in Microscopical and Telescopical observations. Having experienced the great utility of Green and Red lenses, in developing vegetable structures that required to be examined with high powers, I was anxious to derive from this new principle all the advantages which it appeared to possess. In attempting to do this, it became necessary to ascertain the power of giving distinct vision, which belonged to each separate colour of the spectrum, and though I had stated in my former paper, ‘that it was difficult to discover ‘any reason why one coloured medium should be preferred ‘to another, provided each of them transmits equal quantities ‘of homogeneous light;’ yet it was desirable to put this theoretical opinion to the test of direct experiment.