US Description of an Eriometer. [Aug. 



which produce them: and there can be little doubt, from this 

 circumstance, that the globules found in pus are the identical 

 globules of the blood, although probably somewhat altered in 

 the process of suppuration. A minute quantity of the fluid to 

 be examined in this manner may be put between two small 

 pieces of plate glass, and if we hold the glass close to the eye, 

 and look through it at a distant candle, with a dark object behind 

 it, the appearance, if the globules are present, will be so con- 

 spicuous as to leave no doubt respecting their existence. 



II. Description of an Eriometer. 



The rings of colours, which are here employed to discover 

 the existence of a number of equal particles, may also be em- 

 ployed for measuring the comparative and the real dimensions of 

 these particles, or of any pulverised or fibrous substances, which 

 are. sufficiently tfniform in their diameters. Immediately about 

 the luminous object, we see a light area, terminating in a reddish 

 dark margin, then a ring of bluish green, and without it a ring 

 of red: and the alternations of green and red arc often repeated 

 several times, where the particles or fibres are sufficiently uni- 

 form. I observed some years ago that these rings were the 

 larger as the particles or fibres affording them were smaller, but 

 that they were always of the same magnitude for the same par- 

 ticles. It is therefore only necessary to measure the angular 

 magnitude of these rings, or of any one of them, in order to 

 identify the size of the particles which afford them ; and having 

 once established a scale, from an examination of a sufficient 

 number of substances of known dimensions, we may thus deter- 

 mine the actual magnitude of any other substances which exhibit 

 the colours. The limit between the first green ring, and the 

 red which surrounds it, affords the best standard of comparison, 

 and its angular distance may be identified, by projecting the 

 rings on a dark surface, pierced with a circle of very minute 

 holes, which is made to coincide with the limit, by properly ad- 

 justing the distance of the dark substance, and then this distance, 

 measured in semidiameters of the circle of points, gives the cor- 

 responding number of the comparative scale. Such an instrument 

 I have called an Eriometer, from its utility in measuring the 

 fibres of wool, and I have given directions for making it, to Mr. 

 Fidler, in Foley-street. The luminous point is afforded hy a 

 perforation of a brass plate, which is surrounded by the circle of 

 minute holes ; the substance to be examined is fixed on some 

 wires, which are carried by a slider, the plate being held before 

 an Argand lamp, or before two or three candles placed in a line; 

 the slider is drawn out to such a distance as to exhibit the 

 required coincidence, and the index then shows the number 

 representing the magnitude of the substance examined. The 



