^'iles.J 



20 



Mr. Billino-s had not been alone In his belief that this family had 

 Ascidian affinities. M. Koenig considered the Cystidians as Ascidian 

 Mollusca, and so far as regards this family, was supported by McCoy. 

 The features mentioned l)y Mr. Verrill entirely preclude the idea of 

 these fossils being the casts of the interior of Ascidians. The same 

 kind of coverings which Mr. Billings considered as the enclosing sac, 

 sometimes incrusts the Brachiopoda of the same formation. 



Mr. ^S'iles referred to the interest these specimens afford to the natur- 

 alist, and gave a brief review of their scientific history and of the 

 theories of prominent investigators. He then proceeded to show the 

 cystidian affinities of the species by considering the complication of 

 structure exhibited in the group as a type in geological history. He 

 showed that all the features of the genus Cyclocrinites are, at 

 the same time, embryonic and cystidian, and stated that so far as 

 he knev/, this is the only genus of the family yet discovered in 

 America, although the family is well represented in the Palseozoic 

 strata of Europe. 



Dr. B. Joy Jeffries exhibited and explained an optical ex- 

 periment of Prof. Hermann Meyer of Zurich, showing how 

 much our estimation of the distance of objects depends upon 

 the " muscular sense " of the external and internal recti mus- 

 cles of the eyes. 



A series of threads, eight or ten in number, are stretched parallel 

 to each other across a frame about one fourth of an inch apart, and so 

 arranged that the second one is a little nearer and the third still a lit- 

 tle nearer the eye, the fourth and fifth further, the sixth and seventh 

 nearer again, and so on in a zigzag. When these are held before the 

 eyes so that the middle one is about upon a level with them and a 

 foot from them, all the threads seem to be in one and the same plane. 

 Reversing the position of the threads so that they are vertical, they at 

 once appear in the several planes of the zigzag in which they truly 

 are. 



The explanation is, that we are unable to determine the distance of 

 those objects which we cannot bring the optic axes to bear upon so 

 that they cross each other at the object. This Ave can do in regard to 

 any point on a horizontal line, but the line as such we cannot " fix," it 

 will simply appear nearer and thinner, or further and thicker, accord- 

 ing to the degree of convergence of the eyes. If the optic axes are 

 parallel, the eyes being directed straight forward, or if the axes have 

 any degree of convergence, the same simple horizontal line appears be- 

 fore each eye. Now when the threads are vertical, greater effort on the 

 part of the internal recti is required to converge the optic axes upon 



