1898.] MICROSCOPICAL JOURNAL. 59 



roundings, appearing as it does like some crystalline 

 object or laminated mineral .substance wliich has been 

 sectioned at a slight angle with the plane of cleavage. 

 Brilliant blue, orange, and ruby red are the colors that 

 predominate if transmitted light is used, while with a 

 dark ground illumination luminous colors of a light blue, 

 golden and pearly hues appear. The surface of a section 

 of the tapetum made in the above plain appears under 

 higher powers to be finely granular. The granules are 

 arranged regularly in lines which cross each other at 

 right angles giving the appearance of the markings seeu 

 upon some diatoms with a fineness and clearness about 

 equal to Pleurosigma when mounted in styrax. 



The tapetum is circular in shape and has the edge or 

 periphery attached to the interior of the choroid coat of 

 the eye and on a line about one-third the distance from 

 the posterior portion like a veil hung across. 



It does not, however, extend directly across the eye but 

 the free central portion conforms to the globe of the 

 eye so that on cross-section it is irregularly semi-lunar in 

 shape. At the marginal attachment it is very thin but 

 in the central portion it is 1-2500 inch to 1-1600 inch 

 thick. The markings and iridescence do not appear on 

 cross section but they do appear in teased portions of 

 fresh eyes mounted in salt solution or acid glycerine. 



Pigment. — The pigment layer is situated back of the 

 tapetum lucidium, by which on the front it is bounded 

 and on the back by the posterior portion of the choroid. 

 The amount in different eyes varies. In some eyes it 

 apparantly occupies about one-fifth the distance from 

 the posterior to the anterior part of the globe, being 

 1-250 inch thick at the center, while in other eyes it is 

 not more than one-half as thick. The thickness and 

 relative position it occupies to the other parts is best 

 seen in polar sections of the eye, but a better idea of the 

 composition may be had in equatorial sections. It 



