708 SUMMARY OF CURRENT RESEARCHES RELATING TO 



alcohol and glycerine, besides the three just named. By various 

 mixtures of these a series of liquids is obtained, giving any desired 

 index of refraction from 1-333 to 1-606. (Coloration begins when 

 the refraction of the liquid is near that of the object ; when the 

 former greatly exceeds the latter a certain stability of colour appears.) 

 The method is not applicable to bodies opaque in the Microscope, or 

 having too strong colours of their own ; nor yet to bodies having 

 a greater index of refraction than oil of cassia. It may, too, prove 

 difficult sometimes to find a liquid sufficiently indifferent to the object. 

 Herr Maschke indicates how the refractive indices of substances may 

 be compared by his method, and (a more difficult task) numerically 

 determined. He also gives a number of his own determinations. 



Carpenter's * The Microscope and its Revelations.'* — The sixth 

 edition of this book has just been issued. It is revised, and brought 

 down to the present date by numerous additions, including descrip- 

 tions of the more recent forms of microscope-stands, of swinging sub- 

 stages, oil-immersion objectives, and additional accessory apparatus, 

 and the chapter on preparing, mounting, and collecting objects has 

 been re-written. 



In the sections relating to Protophytes, much new matter has been 

 introduced in regard to the Schizomycetes or Bacteria, the Myxomycetes, 

 and other organisms which occupy the border-ground between vege- 

 table and animal life. To the Protozoa large additions have been 

 made, under the heads Monerozoa, Bhizopoda, Infusoria (especially tho 

 flagellate and suctorial), and Badiolaria ; and the section on Sponges 

 has been entirely re-written. Some additions have also been made 

 in regard to the applications of the Microscope to geological inquiry. 



Three important points of microscopical optics are dealt with for 

 the first time in any English treatise on the Microscope. 



1st. An account is given of the modern theory of the estimation 

 of aperture, in regard to which it is pointed out (p. 854) that " Pro- 

 " fessor Abbe's investigation has made it clear that the aperture of 

 " an immersion objective may exceed the maximum of that of a dry 

 " objective." 



2nd. Professor Abbe's diffraction experiments are explained in 

 detail, of which Dr. Carpenter says (p. 188) : " We thus have now for 

 " the first time the scientific rationale of the fact which has long 

 " been practically known, the relation of the ' resolving power ' of 

 " objectives to their angle of aperture." 



3rd. With respect to wide-angled objectives, Dr. Carpenter says 

 (p. 191) that " it is clear that the representations of minute structure 

 " given by objectives of widest aperture are more trustworthy than 

 " those given by those of narrower." 



We have noted a few errata for the next edition : — 



P. 18 : Although Amici, and those who succeeded him, failed to 

 appreciate the importance, as regards larger apertures, of increas- 

 ing the refractive index of the immersion fluid (first embodied in 

 homogeneous immersion), he should nevertheless be credited with 



* 882 pp. (26 pis. and 502 figs.). 



