ZOOLOGY AND BOTANY, MICROSCOPY, ETC 



245 



received on a nicol provided with a convergent lens, two real perfectly 

 distinct foci will appear capable of reception on a screen. The astigma- 

 tism is still more clearly seen with a polarising Microscope. The authors 

 show, however, that an image as sharp as when there is no interposition 

 of a nicol can be always obtained by superposing on the ocular a cylin- 

 drical lens of suitable power, orientated so that the axial section coincides 

 with the plane of symmetry of the prism. 



Cantor Lectures : Theory of the Microscope.* — A series of Cantor 

 Lectures in December and January last were given by C. Beck on the 

 theory of the Microscope. The author did not treat the subject on the 

 usual lines, but devoted his attention mainly to the instrument as at 

 present in actual use, with especial reference to practical considerations. 

 Although he fully recognises indebtedness to others, e.g. E. M. Nelson 

 and J. W. Gordon, his lectures contain much novelty and originality, 

 and will be found to include many points which have recently occupied 

 the attention of microscopists. The first two lectures discuss lenses, and 

 the author gives it as his opinion that the limits of constructive excellence 

 have been practically attained. The third lecture deals with diffraction, 

 and the fourth with practical applications of theory. 



(6) Miscellaneous. 



Compass Reading to gfo or ^Vo Millimetre.!— This instrument 

 (fig. 54) measures objects 3 millimetres thick. The amplification is ob- 

 tained by a lever and a Microscope having at its focus a glass micrometer. 



Fig. 54. 



Caliper with Micrometer Screw.} — This instrument (rig. 55) is 

 mounted on a cast-iron foot, has a ratchet head, and exerts a uniform 



* Journ. Soc. Arts, lvi. Nos. 2875-8 ; and as a reprint. 



t List Phys. and Mech. Instr. Soc. Genevoise, 1907, p. 44. 



% Tom. cit., p. 41. 



