72 PRACTICAL MICROSCOPY. 



upon any object, by simple rotation of the illuminator, an 

 exquisite unfolding of structure takes place. Objects on 

 the slide are brilliantly illuminated, while those on the 

 cover are nearly invisible. 



Another illuminator has been devised by Mr. Wenham, 

 and is shown in Fig. 50. It looks very much like the 

 half of a broken button, which, never- 

 theless, collects and concentrates a sur- 

 prising amount of light. It consists of 

 a semicircular disc of glass J inch in 

 radius, the edge being well rounded and polished to a 

 transverse radius of ^ inch. This may be obtained from 

 Messrs. Ross and Co. and Messrs. Baker. 



We have heard from several microscopists that they 

 have been unable to use this illuminator successfully, and 

 it has generally been the case where it has been mounted 

 for use in the substage. The best plan is to purchase the 

 article unmounted, and to make it adhere, for use, to the 

 underside of the slide, with a little glycerine and gum. 

 This illuminator does not require the addition of any 

 condensing lens, the necessary obliquity of light being 

 readily obtained by swinging the mirror to the side. 

 Mr. Wenham, in writing to the ' English Mechanic,' says 



that while he was never success- 

 ful in the patient manipulation 

 required to bring out the striae 

 of Amphipleura pellucida, yet 

 with the new illuminator he 

 succeeded at once, and on every 

 subsequent occasion. 



FlG The Amici prism ranks next. 



It is made to exhibit one plane 



and two lenticular surfaces, thus concentrating and reflecting 

 the rays at the same time. It is mounted so that it may be 



