473 



purposes of identification, except in the aberrant form of Leptodora 

 hyalina. 



Mr. Ingpen, in introducing the subject of " Iridescence," said 

 his remarks must be considered a stop-gap. At the same time, it 

 might not be improper to call attention to the work of others. 



Mr. Ingpen, remarking that when original papers were scarce it 

 was sometimes worth while to draw attention to published works 

 which might assist in opening out useful and interesting lines of 

 research, gave an account of Dr. Hodgkinson's papers on 

 " Iridescence," which had been communicated to the Manchester 

 Literary and Philosophical Society in April, 1889, and March, 

 1892.* 



The characteristics of minute structures were considered : 



I. Microscopic structures, those that are amenable to ordinary 

 microscopic examination, shown chiefly by pigment or absorption 

 colours. II. Ultra microscopic structures, in which the microscope 

 is used as au aid to the observation of the optical effects of reflec- 

 tion, refraction, absorption, polarization, and various interference 

 phenomena. III. Hyper-photic structures, which from their 

 physical nature are unsuited for investigation by any known 

 optical method, of which lamella3 in optical contact, z.^., separated 

 by intervals of less than a quarter of a wave-length, and trans- 

 parent particles, capable of producing opalescence, are examples. 

 Iridescence was considered to be produced by thin plates, bv fine 

 lines, and by the combination of both structures. In the case of 

 thin plates or films, the light transmitted is in the axis, or parallel 

 to the axis of the illuminating beam, and is coloured. The colour 

 is confined to the direction of the illuminating beam, and changes 

 with the tilting of the film in altitude. In the case of fine lines 

 there is a central colourless image of the source of light, accom- 

 panied by a series of diffraction spectra at right angles to the 

 direction of the lines. These effects are largely modified by dots or 

 irregularities tending to produce opalescence. 



Methods of observation were described. For transparent objects 

 a stage with a pin-hole aperture close below, upon which a one- 

 inch objective was focussed, and means by which the object could 

 be rotated in azimuth, and tilted in altitude, formed a convenient 

 arrangement. For illuminating minute opaque objects a beam of 



* Ser. ir., Vol. ii., p. 193; Vol. v., p. 149. 



