ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 133 



Thirdly, the flagellum of Micrococcus, though but 1/M4,000 in. in 

 diameter, does not come within the range of what I should regard as 

 the minimum visibile with the Microscope. During the past fifteen 

 years we have known of the resolution of lines 112,000 to the English 

 inch. Assuming the lines and the interspaces to be equal {vide Dr. 

 Woodward's photographs of Nobert's 19-band test plate), then they 

 each represent 1/224,000 in. Now, if we have already succeeded in 

 resolving with the Microscope spaces of this degree of proximity, 

 the mere perception or recognition of one single object of 1/144,000 in. 

 is no feat in microscopical manipulation in the direction of the 

 minimum visibile. It would seem that Mr, Nelson, having found a 

 difficulty in exhibiting the flagellum of Micrococcus, has made a 

 random shot at an explanation of his difficulty, and has dropped 

 upon the minuteness as the vera causa, strangely ignoring the con- 

 ditions of visibility by which the object is easily or with difficulty 

 differentiated from the medium in which it is placed." 



Bertrand's Polarizing Prism. — Dr. H. Schroder writes us that 

 the first form of this prism * was devised as long ago as 1869 by 

 Jamin and himself, and later by Feussner.f 



The second form is an impracticable one as no glass is known of the 

 refractive index 1-65 and which has a perfectly white colour and will 

 not tarnish in the air. The angle of field 98° 41' given by Bertrand 

 can only be obtained by moving the eye about from side to side. If 

 the eye is fixed the field is only 47° 27'. If the prism were made of 

 calc spar and cemented with linseed-oil the above angles would be 

 83° 6' and 42° 10'. + 



Bulloch's New Lamp.§— W. H. Bulloch's new lamp is shown in 

 fig. 19. The reservoir and base are similar to those of the " Beck 

 Complete Lamp," but the burner, instead of being in the middle of 

 the reservoir, is placed on one side. This gives room for a brass 

 upright on the other side, which supports the bar carrying the bull's- 

 eye. The latter is focused by sliding the bar in either direction, 

 and the light is directed either upward or downward by swinging 

 the arm as required, and clamping it in any position by the milled 

 head shown. Only one side of the chimney is open, and the 

 rectangular aperture is covered with a plane glass slip 3x1, outside 

 of which may be slipped in a blue slide or one with a ground surface 

 to modify the light. There is also a brass slit, adjustable in width, 

 which fits outside of all. This is intended to give a narrow line of 

 light. The chimney turns about the burner, so that the broad face 

 of the flame or the edge can be used at pleasure. The reservoir, 



* See this Journal, iv. (1884) p. 965. 

 t Ibid., p. 456. 



J See also Zeitschr. f. Instrumentenk., v. (1885) p. 30, from wbi 'h part of 

 Dr. Schroder's remarks are taken. 



§ Amer. Mon. Micr. Journ., v. (1884) p. 205 yl fig.). 

 il See this Journal, iv. (1884) p. 628. 



