4 ANDREWS. 



happy chance also brought within my reach in the conservatory 

 some material specially adapted to certain of the problems. 



Thanks are due also the great courtesy shown by M. Lacaze 

 Duthiers, by whose permission I was enabled to enjoy, in the 

 summer of 1894, the unique privileges of the Zoological Labo- 

 ratory at Roscoff, France, and there to repeat, under most 

 favorable circumstances, the observations on starfish and echi- 

 nus eggs ; upon which facts, because of their very startling 

 nature, I should otherwise have been loath to build from one 

 series of observations. 



Methods.^ 



The optical instruments were, with one exception, of Zeiss ; 

 a Zeiss stand, largest size, and accessories ; 4 mm., and 2 mm. 

 immersion, objectives; compensating eyepieces, 4, 6, 8, 12. The 

 exception spoken of was a picked ^^ immersion of Beck's. 

 The work done with this was afterward verified with the Zeiss 

 tools. The illumination was from an Abbe condenser, with at 

 night blue glass, and a bull's-eye lens, or waxed-paper screen. 



Considering the nature of the work, it should possibly be 

 mentioned in this connection that it was further assisted by 

 exceptionally far-sighted eyes, having great range and swiftness 

 of accommodation. 



The optical conditions were found to be of utmost impor- 

 tance. Pure light on a clear day, from a northern exposure, 

 gave best results. Failing this, good artificial illumination 

 with above conditions was preferred, with precautions against 

 undue heat. 



The work done with higher combinations was always com- 

 pared, where possible, with results given by a series of lower 

 powers ; that done with lower powers was verified always under 

 higher combinations ; so that optical appearances were thus 

 compared with their physical basis of structure. This method 

 gave often important results. 



For the more delicate structures and phenomena a sensitive 

 adjustment of optical conditions was found to be essential, and 



^ For the reader's convenience the leading results are indicated by numbers in 

 brackets. 



