U2 



THE AMERICAN MONTHLY 



[August, 



without the least organization. From 

 them have been formed, by the dif- 

 ferentiation of an^intestine in jthe in- 

 terior, unicellular protista, cellules 

 of elementary simplicity, amorphous 

 and indifferent, resembling the am- 

 oebse. Some of these unicellular 

 protista being possessed of social in- 

 clinations, are accustomed to live 

 united in small colonies; the earliest 

 polycellular organisms appeared first 

 as simple cellular aggregations, asso- 

 ciations of homogeneous cellules 

 united by very lax bonds. It is, in- 

 deed, probable, that these very an- 

 cient beginnings of the evolution of 

 organic life were repeated in many 

 different parts of the earth, then in 

 its youth, at the same time and in an 

 independent manner. In this way di- 

 vers, and it may be many, varieties 

 of protista may have been born iso- 

 lated from each other, first unicellu- 

 lar, afterwards polycellular. Owing 

 to the struggle for existence, which 

 began even among the protista with 

 the aurora of life, these creatures ad- 

 vanced, little by little, to a superior 

 grade of differentiation and perfec- 

 tion. The most important advance 

 was indisputably the reciprocal sepa- 

 ration of the vital processes of ani- 

 mals and plants. Among these pro- 

 tista, some began to adopt themselves 

 to a mode of living which has become 

 that of animals, others to a kind of 

 existence which is that of vegetals, 

 and the characteristic form of the 

 bodies of these creatures resulted 

 from these adaptations. A third group 

 of protista, a conservative group, 

 maintained its original neutral char- 

 acter. While these divers adapta- 

 tions were becoming fixed m the 

 course of centuries by heredity, the 

 three grand organic kingdoms were 

 completing their development. 



Ernst Oundlacli's Substage Re- 

 fractor. 



This apparatus, which is designed 

 for the measurement of the angular 

 apertures of vvide-angled objectives, 



consists of a small crown-glass cube 

 with sides of about -jpg- of an inch. 

 One of its surfaces is made opaque, 

 the one opposite to this, and also the 

 two others opposite each other, are 

 polished. The cube is made to ad- 

 here, by means of a suitable homoge- 

 neous medium, to the front surface of 

 the objective the angular aperture of 

 v.'hich is to be determined, by the pol- 

 ished surface opposite the opaque 

 side. Then a ray of light must enter 

 each of the polished side surfaces in 

 the plane described by the optical 

 axis of the objective and a line 

 perpendicular to those polished sur- 

 faces, and at such angular inclina- 

 tion to the optical axis that it 

 will pass through the objective close 

 at the edge of its aperture, and emerge 

 from it in the direction of the optical 

 axis. 



The angle described by the refrac- 

 ted rays inside of the crown-glass 

 cube, is equal to the crown-glass an- 

 gle of aperture of the objective and is: 



a 



cos. ;/ = cos. 



r 



a being half the angle described by 

 the two light-rays before entering the 

 crown-glass cube, r the refractive in- 

 dex of the crown-glass, of which the 

 cube is made, /i the crown-glass an- 

 gle of the objective. 



Fig. 35. 



The accompanying diagram may 

 serve to make the principle more 

 easily understood: c is the front lens 

 of the objective; d the crown-glass 



