314 CARNEGIE INSTITUTION OF WASHINGTON. 



same; but the leaf would acquire a more plastic character, fitted to 

 more varied enviromnents, as in the Cretaceous Liriodendropsis. 

 Lobing, clefting, and dividing could readily follow, and, with develop- 

 ment of opposite basal veins, trifid types and also palmate forms. 



Moreover, there is no evidence of fusion of cycadeous pinnules ; the 

 spinose cycadeous types Uke Dioon and Encephalartos are in accord 

 with their position in time, terminal, or extreme forms. A great 

 extent of ginkgoid foUage in the Rhatic and variety in cycadeoid 

 foliage in the Jura are two great developments preceding the appear- 

 ance of palmate forms in the Potomac. But palmate leaves need not 

 be regarded as more distinctly primitive than bladed forms ; and double 

 as opposed to single leaf- traces must be regarded as having been 

 anciently separated and distinct from the Paleozoic on. From these 

 cognate facts it appears that with the discovery of but few additional 

 forms the actual record of pre-Cretaceous dicotyl leaf and stem devel- 

 opment may be brought into view. 



PHYSICS. 



Barus, Carl, Brown University, Providence, Rhode Island. Continuation of 

 investigations in interferometry. (For previous reports see Year Books 

 Nos. 4, 5, 7-16.) 



Professor Carl Barus has recently submitted a report* of an investi- 

 gation of methods of research by which displacement interferometry 

 with the aid of the achromatics discussed in the preceding report,* 

 gives promise of fruitful appHcations. In Chapter I the method of 

 measuring small angles hitherto suggested is given a practical test; 

 the general theory of the subject in its bearing on the two possible 

 methods is developed at some length, and a variety of interferometer 

 devices, with mirror, ocular, and collimator micrometers, are instanced. 

 In connection with these experiments a method of reducing the 

 fringes to the smallest number possible, practically to a single fringe, 

 is elucidated. 



As the achromatic fringes can not (in general) be found without 

 first finding the corresponding spectrum fringes; and conversely, since 

 for each type of spectrum fringes (direct or reversed), a corresponding 

 group of achromatic fringes may be associated. Chapter II treats of 

 spectrum fringes differing in their manner of production. The en- 

 deavor has been to obtain interferences from distant slender luminous 

 objects, without the aid of a slit. Partially, at least, the work has 

 succeeded. 



The work in the third chapter was undertaken at the request of 

 Professor W. G. Cady, of Wesleyan University, in an endeavor to 



* Carnegie Inst. Wash. Pub. No. 249, part II, chapters V, VI (1917). 



