CAXADr.W Flf^flEJilEH EXPEDITIOX, 191 ',-1} 



83 



scale; these divide tlie striped iiortiou of the scale into nine zones, each havin/r the 

 outline of a horseshoe; in plate I, fij;. 1 these appear lighter than the narrow lines. 

 By altering the light in which they are viewed, these broad zones can be made to 

 appear darker than the narrow lines (plate 11, fig. 2). These latter are the winter 

 rings of the scale, and the broader zones representing summer growth; it is by count- 



Plate I. 



ing these that the age of the tish is determined, and by measuring the distance between 

 them, it is possible to calculate the growth. 



It is an easy matter to show that all the details above referred to pertain to the 

 outer surface of the scale, i. e., that surface of the scale which faces outward when 

 the scale itself is in its normal position on the body. The optical effect is a result 

 of reflection and refraction in this external surface. The demonstration may most 

 easily be made by producing an impression of the outer surface of the scale in a 



