Biographical Memoir ofDr Thomas Voimg. 229 



colours, with which every strong light seems to be surrounded 

 when it is looked at through a mass of floating motes, or of minute 

 filaments which are of equal dimensions. It was these rings in 

 truth, that suggested to Young the idea of an extremely simple 

 instrument which he called Eriometer, and with which we can 

 measure without difficulty the dimensions of the most minute 

 bodies. The Eriometer, which is yet but little known among 

 observers, has an immense advantage over the microscope, in 

 that, in a moment, it supplies us with the mean size of the mil- 

 lions of particles which are found in the field of vision. It, 

 moreover, possesses the singular property of giving no indica- 

 tion when the particles differ very much among themselves, and 

 when the determination of the question could be of no real bene- 

 fit. Young applied his eriometer to the measurement of the 

 globules of the blood in the different classes of animals, — to 

 that of the fine dust of various species of vegetables, and also 

 to measure the fineness of the fleecy fibres employed in the ma- 

 nufacture of goods, from that of the hair of the beaver, the finest 

 of all, to that of the fleece of the sheep of the Sussex breed, 

 which was placed at the other extremity of the scale, and the 

 filaments of which were four and a half times as thick as the 

 hair of the beaver. 



Previous to the time of Young, the numerous phenomena re* 

 specting coloration, to which we have been alluding, were not 

 only inexplicable, but could not in any way be associated with 

 each other. Newton, for example, who was so long engaged 

 with the subject, did not see any connection between the irises 

 of these laminae, and the colours of the spectrum. Young 

 shewed that these two kinds of coloured striae were only diffe^ 

 rent effects of interference. At a subsequent period, when 

 chromatic polarization had been discovered, he deduced from 

 some measurements of thickness, some remarkable numerical 

 analogies, which led to a strong presumption that, sooner or 

 later, this extraordinary kind of polarization will be connected 

 with his doctrine. Before that takes place, however, we must 

 allow, there is a great void to fill up. Important properties of 

 light which were then wholly unknown, did not allow the sup- 

 position that in certain crystals, &c. double refraction would 



VOL. XX. NO. XL. APRIL 1836. Q 



