structure of the Scales of Lejpisma Saccharina. 13 



a silicated base. In this case tlie silica has consohdated into 

 individual sub-crystalline grains before the other minerals, whereas 

 in all true granites the silica has been the last to consolidate. The 

 presence of aqueous (?) vapour during the consolidation of this rock 

 is shown by the existence of numerous fluid cavities, and is another 

 feature in which it resembles true granites. Other quartz-porphyries 

 which I have examined show cavities in the silica, but generally 

 destitute of fluid bubbles. 



IV. — The Structure of the Scales of Lepisma Saccharina. 

 By Gr. W. Morehouse. 



For many years this test has been subjected to most careful and 

 critical examination by the most competent observers and with the 

 best microscopes, but, after all, the true character of its markings 

 still remains a disputed question. These differences of opinion 

 have evidently arisen partly from the complex nature of the mark- 

 ings themselves, and partly from the different conditions under which 

 they have been seen. In this scale we have coarse ribs easily seen 

 with a very ordinary glass, and on the other hand delicate struc- 

 tures severely taxing the powers of the finest objectives in existence. 

 This fact alone is sufficient to account for the want of agreement, 

 without accusing any person of being biassed by a theory ; while 

 those observers who think their own instruments are the best will 

 continue to be satisfied with what they may happen to see, and 

 shut their eyes to any advance. 



As the microscope has been improved, our ideas of the structure 

 of the Lepisma scale have been gradually modified, and who will 

 now claim it to be " too easy for a test object " ? 



In the order of difl&culty of resolution we have — 



1. The heavy longitudinal ridges running from end to end of 

 the scale and slightly projecting at the point. 



2. Distinct ribs generally radiating from the quill, or curved 

 parallel with the outline of the scale, and becoming faint in the 

 centre and parts remote from the quill. 



3. Transverse corrugations of the membranes. 



4. Faint irregular veins branching from the diverging ridges 

 (No. 2) generally taking a transverse direction, and, together with 

 the corrugation, causing the spurious appearance of fine beading 

 at their points of intersection with the ridges. 



To make sure of my work on this scale I have studied it under 

 a number of different conditions. The observations have been 

 conducted with monochromatic sunlight ; with white cloud and 

 lamp ; with central beam and oblique ; with mirror, prisms, achro- 



