ZOOLOGY AND BOTANY, MIGROSCOP\, ETC. 



449 



a dark-ground condenser. It enables one to watch the object through 

 the side eye-piece and to defer the exposure until a favourable moment 

 presents itself. 



Magnifying- Power of Eye-pieces.* — E. M. Nelson gives the follow- 

 ing list of magnifying powers of some modern eye-pieces — the Powell 

 and Lealand A eye-piece, which has an equivalent focus of 2 in., being 

 inserted as a sort of fiducial standpoint : — 



Winkel No. 2 Huyghenian . . . . 30 



Powell and Lealand A .... 32 



Leitz No. 4 compensating .. .. 33 



Winkel No. 3 eomplanat . .. 45 



,, No. 4 eomplanat .. .. 60 

 Zeiss No. 12 compensating long 



tube 72 



Winkel No, 6 compensating . . 88 



Winkel No. 5 eomplanat 

 Zeiss No. 18 compensating long 

 tube 



Leitz No. 12 compensating .. 

 Zeiss No. 27 compensating long 



tube 



Leitz No. 18 compensating .. 



90 



96 



98 



146 

 153 



(3) Illuminating and other Apparatus. 



Correction of Errors of Refraction for Microscope Work.t — The 

 eye-strain which not infrequently I'esults from ]:>rolonged use of the 

 Microscope, especially when working with high powers and artificial 

 light, is, says W. B. Leishman, often so great as to cause considerable 

 discomfort and headache, and may even lead 

 to the abandonment of microscope work, ex- 

 cept for brief examinations. In many cases 

 this trouble is caused by errors of refrac- 

 tion, more particularly by some degree of 

 astigmatism. If this astigmatism is con- 

 siderable, the microscopist is practically cer- 

 tain, in these days, to be aware of it, and 

 to possess glasses which correct his particular 

 error, but if it is small it may never be de- 

 tected until advancing years lead him to con- 

 sult an oculist as to his first pair of presbyopic 

 glasses. In either case, when he attempts 

 to work at his Microscope with spectacles or pince-nez in situ, he is 

 certain to find them so uncomfortable and inconvenient that, sooner or 

 later, he discards them, and trusts once more to his unaided vision and 

 his powers of accommodation, with the frequent result that continuous 

 work becomes increasingly difficult and the effects of eye-strain more 

 conspicuous. 



The small device here illustrated (fig. 79) has been designed with a 

 view to correcting the error of refraction without employing spectacles. 

 It is so obvious and simple that it is very probable that something similar 

 may have been described and used long ere this, but, since the writer 

 has been unable to discover that this is the case, it appears worth while, 

 for the sake of others similarly situated, to describe the ocular cap which 

 he has had made for his own use. The increased definition which has 

 resulted from the use of this cap is unmistakable, and there has also 

 been a marked lessening of the feeling of strain which used to result 

 from long hours of high-power work. 



* English Mechanic, xcv. (1912) p. 591. 

 t Brit. Med. Journ. (1912) i. p. 123 (1 fig.). 



