710 



SUMMABY or CURRENT RESEARCHES RELATING TO 



in tte diaphragm of the condenser, and in an iris diaphragm fitted 

 on to the lower portion of the condenser between it and the prism, a 

 most exquisite image of the tree was seen. The definition of this 

 was charming, every little twig and incipient bud being distinctly 

 visible. Then, nothing else being altered, a plane mirror was sub- 

 stituted for the prism. " What a change ! The larger branches were 

 there indeed, but the slender twigs were involved in hopeless ' fuzz,' 



which no amount of manipulation 

 Fig. 14S. could eliminate." The experi- 



ment was varied by forming the 

 image of a net window-curtain 

 about three yards distant from 

 the Microscope. With the prism, 

 the picture of the network and 

 pattern was perfect, every detail 

 being exquisitely shown. With 

 the plane mirror, the image was 

 very markedly inferior, though 

 less so than in the former ex- 

 periment. 



Zentmayer's Abbe Con- 

 denser.* — A simple and inexpen- 

 sive mounting for the Abbe con- 

 denser (shown in fig. 148) has 

 been devised by Mr. Zentmayer, by means of which it can be used 

 with any substage. The milled head, seen below on the right, moves 

 the plate which carries the diaphragms. 



Topler's Illuminating Apparatus.! — In the interior of micro- 

 scopical objects many parts escape observation, not only on account of 

 their small size, but also because very frequently their density differs too 

 little from that of their surroundings, and consequently they influence 

 but slightly the path of the rays. Dr. A. Topler drew special atten- 

 tion to this subject in 1864, J when he described an apparatus called 

 by him " Schlieren " (streaks) apparatus, on account of its use for the 

 examination of streaks in glass, a (fig. 149) is a point of light sending 

 rays to the lens p q ; these will be refracted to &. To an eye df, which 

 receives all these rays, and is so accommodated that it clearly sees 

 the lens, the latter will appear brightly illuminated. If, however, a 

 diaphragm c A is moved towards the point h, then at the moment that 

 it passes the point the rays will be entirely shut off and the lens will 

 appear dark. If, however, there is a more strongly refracting point in 

 any part of the lens, e. g. in g i, the rays, passing through this point, 

 will not meet the axis at b, but nearer to or further from the lens, or 

 will not meet it at all ; these rays will then pass by the side of h. 

 When the diaphragm is moved forward it will cut off part of the rays 

 before the normal rays are affected, and the spot in question will 



* Amer. Mon. Micr. Journ., vi. (1885) p. 84 (1 fig.). 



t Zeitschr. f. Instrumentenk., ii. (1882) pp. 92-6 (3 figs.). 



j 'Beob. nach einer neueu optischen Methode,' Bonn, 1864. 



