HA RD WICKE' S SCIENCE- G OSS IP. 



101 



stop H (Fig. 59), which is also of metal, and which is 

 joined to the carrier by a fine steel wire, K. The 

 carrier slides in a square nose-piece, L (Fig. 59), 

 between the objective and the body tube of the 

 microscope, as shown at the double-dotted line N 

 (Fig. 59). The nose-piece should have a revolving 

 fitting, as shown in the sectional view. The handle 

 of sliding carrier is at I (Fig. 59). The stop H 

 (Fig. 59) must be of the same size as the image of the 

 mirror in the objective used, and is for the purpose of 



Fig. 59- 



Fig. 60. 



intercepting this image in the objective. M is the 

 objective, and O is the standard screw for body-tube 

 (Fig. 59). 



It will easily be seen that by moving the stop the 

 image of the mirror can be intercepted at any point 

 from the centre to the extreme edge of the objective. 

 The best effects are obtained when the stop is placed 

 a little beyond the centre, as at C (Fig. 54). 



The motions indicated can all be reversed by means 

 of the revolving fitting of the nose-piece. Changes 



from dark field to bright, and the reverse, can be 

 instantly made by sliding the carrier in its fittings. 



The effects obtained by the use of this piece of 

 apparatus with homogeneous-immersion lenses are 

 very remarkable. The internal organs of infusoria 

 are shown with a precision and beauty never equalled. 

 Bacteria in fluids are seen as brilliant points of light. 

 Vast numbers that are above and below the focus, 

 and which could not be seen in a bright field, and 

 brought into view. The trachea of mosquito larvce 

 can easily be traced as beautiful thread-like lines 

 throughout their entire length. 



In examining stained human muscle containing 

 trichina, and using a one-fifths dry objective with 

 bright field, great care is required to see the parasite 

 in its cyst, but, on a dark field produced by the use of 

 the stop, the worm will be seen as a brilliant coil, and 

 can be plainly traced from tip to tip. — American 

 Monthly Microscopical Journal. 



THE BEST HUNDRED BOOKS IN 

 NATURAL HISTORY. 



By H. Roberts. 



THE list given under the above heading, in the 

 February number of Science-Gossip, has 

 elicited various suggestions and proposed alterations 

 from readers of this journal. For the most part 

 these are not of very great value ; but some hints 

 which I have received have been most useful, and 

 my very best thanks are due to all those who have 

 so kindly tendered advice whether I have followed it 

 or not. 



The following is a list of useful alterations : — 

 The " Classical " and " General Natural History" 

 lists require no alteration. The same is the case 

 with the "Handbooks" "Works on Prehistoric 

 Times," "Atlases," and "Biographies." In the 

 "Works on Function" Lubbock's " British Wild 

 Flowers" and " Flowers, Fruits, and Leaves," may 

 be omitted. 



The following list of "Text Books'' may replace 

 the previous one with advantage : — 



Text Books. 



Huxley and Martin's " Practical Biology," or 

 Marshall and Hurst's " Practical Zoology;" Claus 

 and Sedgwick's "Zoology;" Gray's "Human Ana- 

 tomy;" Flower's " Comparative Osteology ; " Hux- 

 ley's or Foster's" Physiology ; " Foster and Balfour's 

 or Balfour's "Embryology;" Tylor's "Anthropo- 

 logy ; " Lyell's " Elements of Geology ; " or Geikie's 

 "Text Book of Geology;" Oliver's "Elementary 

 Botany," or Prantl and Vine's "Text Book of 

 Botany;" Goebel's "Morphology of Plants:" 

 Sach's or Vine's "Physiology of Plants;" Bower's 

 "Practical Botany." 



