ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 689 



it necessary to fix the Microscopes successively in all possible posi- 

 tions with regard to one another, for each position to set all the 

 divisions in succession under one Microscope and make readings in 

 all the others. With four Microscopes and seventy-two arcs of 5° 

 each, this would involve six million readings. 



Dr. Schreiber's simplified method is as follows : — Fix the Micro- 

 scopes at certain equal distances corresponding to certain arcs ; bring 

 one division into the first Microscope A, and measure micrometrically 

 the distance of the division seen in each of the Microscopes from its 

 zero point. These readings form a " set." A second set is got by turn- 

 ing the disc until the next division comes into A ; observe all the 

 arcs in this way, then all these sets form a u series." Thus a series 

 consists of as many sets as there are arcs, and a set of as many 

 readings as there are Microscopes. 



It is impossible to abstract the details given by the author, for 

 which reference must be made to the original paper. He finally gives 

 " schemes " or arrangements of the observations for the following 

 three cases ; (1) Two Microscopes ; (2) four Microscopes which can 

 be fixed in any positions ; (3) four Microscopes fixed in pairs opposite 

 to one another ; and compares the number of readings which they 

 involve, from which it appears that method (2) is the most 

 advantageous. 



Measuring 1 the Focal Length of a Lens.* — Prof. E. Lommel 

 adopts the following method : — At the point in the tube of an eye- 

 piece O (fig. 145) generally occupied by the cross- wires, a semi-circular 

 screen is fixed which obscures half 

 the tube, the screen being divided Fig. 145. 



into two quarter-circles by a narrow 

 vertical slit. Behind this is a mirror 

 or prism which sends light from an 

 opening o in the side of the tube 

 through the slit and into the lens L, 

 which is so placed that its axis 

 coincides with that of the eye-piece ; 



behind the lens is a plane mirror S which reflects the light back 

 through it. The distance between lens and eye-piece is altered until 

 the image of the slit appears sharply defined, and without parallax, 

 as a prolongation of the slit itself. The distance between the lens 

 and slit will then be the focal length, since the rays are in this case 

 refracted through the lens as a parallel pencil, reflected back as 

 parallel rays, and converge again to the principal focus at the position 

 of the slit. This length is most conveniently measured by fixing the 

 lens and the eye-piece upon stands which slide upon a graduated bar. 



Measuring Indices of Refraction.! — Prof. Lommel also de- 

 scribes the following method : — The telescope F (fig. 146) of a spectro- 

 meter, fixed and focused to an infinite distance, is provided with the 

 eye-piece o described in the preceding note ; the prism P whose 



* Zeitschr. f. Instrumentenk., v. (1885) p. 124 (1 fig.). 

 t Ibid., p. 125 (1 fig.). 

 Ser. 2.— Yol. VI. 2 Z 



O 



