IIAIUS, FEATHERS, AND SCALES. 



21 



tially worn down by rubbing against the gravel of the 

 bottom, or against other objects in the water. Besides 

 the concentric lines, there are seen on many of the scales, 

 especially those of the lateral line, radiating lines varying 

 in number from one to twenty, or more, diverging from 

 the centre towards the circumference, and frequently 

 connected by cross lines forming a sort of net-work 

 around the centre (see c). Under the microscope, these 

 lines appear to be elevated ridges, dividing the con- 

 i fiitric lines ; but of their use I am ignorant. 



What I have just stated is the ordinary explanation of 

 these hoe concentric lines ; but a careful examination of 

 the structure with much higher powers than we have been 

 using, induces me to doubt its correctness. Reverting to 

 the scales of the Perch, 

 let us notice the clear 

 diverging bands, which 

 look as if the whole scale 

 had been split in several 

 places, and the openings 

 thus made filled with 

 uniform clear substance. 

 The same structure is 

 seen in many other 

 scales, as in this cycloid 

 one from the Flounder 

 which, beinof coarse! v lined, shows the structure well; or 

 i n these from the Green Wrasse. I will now apply to one 

 of these a power of 600 diameters, concentrating the light 

 thrown through the scale from the mirror by the achromatic- 

 condenser,* and examine the scale anew. You now see 

 two distinct layers : the upper one which bears the con- 

 centric lines, and a lower clear one which not only fills the 



' Called achromatic, from its being so constructed as not to split the 

 li'dit coming through it and cause colour. The term is tunned from 

 the Creek a, k<>L Yptoua (chroma), colour. 



SCALE OF FLOUNDER. 



a. Natural size. 



