172 The Microscope. 



under date of August 6th, 1886, in "Science," calls it the^best 

 treatise on the organization of the cell ; and evidently the ap- 

 pearance of this work marks an era in the science of Cytology. 

 But since the appearance of this treatise much has been done in 

 Cytology by Canon Carnoy and by his disciple M. G. Gilson, and 

 their observations and researches have been published in the 

 revue called " La Cellule." 



I reserve for myself to speak of these in my concluding article. 

 For as I have chosen Prof Carnoy as guide in these studies, and 

 for the most part let him speak for himself, I wish to follow him 

 also to the end of his studies. By thus making known his re- 

 searches to the American public, the author of these papers 

 believes that he is repaying with gratitude the kindness shown 

 to him by the professor, whilst studying under him in the Cyto- 

 logical Laboratory at Louvain during the years of 1881 and '82. 



THE POLARISCOPE. 



JOHN M. HOLZINGER, PH. D. 



A STICK thrust obliquely into a vessel appears abruptly 

 broken at the point of contact with the water. A penny 

 lying in an empty dish and just concealed by the side of the 

 dish from one looking obliquely into the bottom, will come into 

 view if the dish be filled with water. Light travels in straight 

 lines, but only so long as the medium is of uniform density, 

 or so long as the light falls perpendicularly on media of dif- 

 ferent density. An oblique ray is bent from a perpendicular 

 passing into a rarer medium ; on passing to a denser medium, 

 it is defected toward that perpendicular. And the above well 

 known phenomena illustrate refraction. 



Now, all the transparent substances met with in our experi- 

 ence, as water, air, glass, do thus deflect the rays of light. Some 

 less common transparent solids, always crystalline, modify light 

 in a more unusual way. If we could replace the water in the 

 dish by Iceland spar, the penny would again appear above the 

 edge, but it would now seem to be doable. This mineral thus 

 possesses the property of double refraction. The two rays, re- 

 sulting from the splitting up of one, emerge from the crystal 

 istill more remarkably modified ; both now are polarized, the one 

 following the ordinary law of refraction being known as the 



