OPTICS. 



37 



figure, till one or more of the reflected 

 images are distinctly separated from the 

 bright image seen by transmitted light 

 in the direction R V, the reflected image 

 will be crossed with about 15 or lo 

 beautiful parallel fringes. The central 

 fringes and the external ones on each 

 side, have the same relation to each 

 other as those formed by thin plates. 

 The direction of the fringes is parallel 

 to the common section of the reflecting 

 surfaces ; and Dr. Brewster determined 

 by a series of experiments, that their 

 production depends upon the action of 

 all the four surfaces of the plate of 

 glass, and that their magnitude is in- 

 \-erseIy as the thickness of 'the plates that 

 produce them at a given inclination. 



These results are explicable by the 

 law of interference, the effect of the in- 

 clination of the plates being, as Dr. 

 Young has shown, to reduce the vertical 

 thickness of the plate in the ratio of the 

 cosine. See Edinburgh Transactions, 

 vol. vii. p. 435 144. 



Colours of Double Plates of Unequal 

 thickness. In using a sextant, Mr. 

 Nicholson observed colours on the 

 glasses employed for the sights, and he 

 regarded them as analogous to those 

 of thin plates. Dr. Young considered 

 them as arising from a slight difference 

 in the thickness of the two plates, and 

 as the same that would be produced 

 by a single plate whose thickness is 

 equal to the difference of the thickness 

 of the plates. 



CHAPTER XV. Colours of Minute 

 Particles and Fibres Eriometer. 



IF we look at the sun, or a candle, 

 through a plate of glass upon which 

 we have gently breathed, or over which 

 we have scattered particles of dust, or 

 of any fine powder, we shall observe it 

 surrounded with rings of colours. By 

 using the seed of the Lycopodium, or 

 by placing a drop of blood dilute:! with 

 water between two pieces of glass, the 

 rings of colour will be finely exhi- 

 bited. Round the luminous body there 

 is seen a light area, terminating in a 

 reddish dark margin ; this is succeeded 

 by a ring of bluish green, and then by 

 a red ring, these two last colours suc- 

 ceeding each other several times when the 

 particles are of an uniform diameter. 



As the diameter of the rings thus 

 produced increases when the particles 

 or fibres become smaller, Dr. Young 

 proposed to measure the diameters of 

 such minute bodies, by determining the 



size of the rings which they produced 

 For this purpose he selected the limit 

 of the first green ring and the red one, 

 and by means of an instrument which 

 he crJls an Eriometer,* he was enabled 

 to measure the size of minute particles 

 or fibres. This instrument consists of a 

 plate of brass, having an aperture in 

 its centre of about the sixtieth of an 

 inch in diameter. This aperture is sur- 

 rounded by a circle of perforations 

 about half an inch in diameter, the per- 

 forations being 8 or 10 in number, and 

 as minute as possible. The eye being 

 aided with a lens, the substance to be 

 examined is fixed in a slider, and the 

 instrument being held before an Argand 

 lamp, or two or three candles placed in 

 a row, the slider is drawn out till the 

 limit of the first green ring and the red 

 one coincides with the circle of perfora- 

 tions, and the index shows on the scale 

 the magnitude of the particles or fibres. 

 In order to find the value of an unit 

 on this scale, Dr. Young availed him- 

 self of an observation of Dr. WoUaston, 

 that the seed of the Lycoperdon Bovista 

 was the 8500dth of an inch in diameter. 

 This powder gave rings in which the 

 limit of the first green and red indicated 

 3k on the scale ; so that the value of an 

 unit of the scale was 3^ times 8500, or 

 the 29,750th part of an inch, or, in round 

 numbers, the thirty thousandth part of 

 an inch. The leading results obtained 

 by Dr. Young are found in the follow- 

 ing Table : 



Parts of the scale 



Milk diluted, very indistinct 3 



Dust of Lyioperdon Batista, very distinct .,.. 3.5 



Bullock's Blood, from Beef 45 



Fibres of crystalline lens 5.5 



Smut of Barley, called male ear 6. J 



Blood of a Mouse 6.5 



Human Blood diluted with water, 5; after 



standing some days, 6 or 7 



Blood recently diluted with serum only 8 



Pus .. 7.5 



Silk, very irregular, about 1^ 



Beaver Wool, very even, (jointed) 13 



Angola Wool, abour. M 



VignniaWool 1 



Siberian Hare's Wool, Scotch Hare's Wool, 



Foreign Coney Wool, yellow Rabbit's Wool, 



about " ." 13.5 



Mole's Fur, about 16 



Skate's Blood, very indistinct, about 16 



American Kabplft Wool, British Coney Wool, 



about 16.5 



Buffalo's Wool 18 



Woo! of the Ovis .Montana 



Finest real Wool, mixed, about 1^..~> 



Shawl Wool, is ur 19 



Goal's Wool 19 



n, very unequal, about I'.l 



Peruvian "Wool, mixed, 'lie finest locks 



A email lock of Welsh Wool '20 



S.ixon Wool, a few fibres, 17, some 23, chief 1 -. 

 Wool of sin Escurial Ram, at Lord Somerville's 



show, 23 to 24 



* From two Greek words which signify a measure 

 and wool. 



