ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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of which the lens I throws the rays coming from the slits parallel upon 

 the prism system. The long axis of the box is at an angle of 30° with 

 both collimator and telescope B. By the objective I' of the lattter a real 

 spectrum of the two slits is thrown in the plane i, which is observed 

 under a magnification of 20 times by the lens L contained in the tube B'. 

 The apparent magnitude of the spectra thus exceeds by about 4 times 

 that of the spectrum in the microspectral-ocular of Abbe-Zeiss, and by 

 8 times that of the Sorby-Browning ocular. Projected to a distance of 

 250 mm., the distance of the Fraunhofer lines a and g amounts to 185 mm. 

 Except for observations on the extreme red and violet, the intensity 

 when using gaslight is sufficient to allow of the use of the strongest 

 immersion system. With a slit of less than 0'025 mm. the spectrum 

 of the sun's light after passage through two ground glasses showed the 

 D line doubled, with one line clearly broader and darker than the other. 



Fitted into a third opening in the prism-box is a seccmd collimator- 

 tube C carrying at sc an Angstrom's scale of wave-lengths (bright lines 

 on a dark ground) which is illuminated by means of a mirror S'. When 

 not in use, a movable screen d' is pushed over the opening of C. The 

 light rays from the scale rendered parallel by the lenses I" and I'" are 

 reflected into the telescope from the end face of the prism system, and 

 an image of the scale is formed by the lens V in the plane ^. 



To obtain the proper position of this image with respect to the 

 spectra, the tube C is movable to a limited extent in the box A', so that 

 the direction of its axis to the end-face of P can be altered. For this 

 purpose, C is fastened to the metallic arm m', which is pressed by means 

 of a spring v against the screw w. By turning the latter the correct 

 position of C is easily obtained. 



Finally, the tube B is provided with two pairs of diaphragms movable 

 in the plane i at right angles to each other. One pair is used in order 

 to limit the spectra to the particular group of wave-lengths under 

 examination. The edges, which run 



parallel to the Fraunhofer lines, are Fig. 17. 



adjusted by the screws t and t'. 

 The other pair, movable by the 

 screws u and u' (seen in fig. 16) 

 serve to make the two spectra of 

 the same breadth, for experience 

 shows that, in order to compare 

 two spectra, tliey should be of the 

 same form and size. 



Powell and Lealand's Apochro- 

 matic Condenser. — Following upon 

 the extension of the apertures of ob- 

 jectives due to what Prof. Abbe has 

 termed " Stephenson's homogeneous- 

 immersion system," Messrs. Powell 

 and Lealand have brought out the 

 Apochromatic Condenser of 1 • 4 N.A., 

 shown in fig. 17. This extended aperture involves the employment of a 

 combination of lenses of such large diameter, that it was not found prac- 

 ticable to utilize the usual revolving disc of diaphragms, stops, &c. ; 

 hence the application of a pivoting diaphragm-carrier that can be slid 

 up in close contact with the posterior lens of the condenser, the pivoting 



