526 



SUMMARY OF CURREKT RESEARCHES RELATING TO 



Insect Cage.— This (shown in Fig. 127) is intended for the study 

 of live insects. To one end of a 3 by 1 brass plate is attached an 

 upright, supporting a cross-piece at the top on which rests the stem 

 carrying the cage. This is held in place by a spring, and may bo 

 rotated or moved longitudinally by a milled head. The cage is con- 

 structed on the same principle as the ordinary live-box, but instead of 



Fig. 127. 



glass, bobbinet is used in order to confine the insect better and with- 

 out injury. A cover of glass can be used if preferred. The cage 

 may be rotated in the optic axis of the Microscope in order that the 

 object may be illuminated to the best advantage. 



The Essence of Homogeneous Immersion. — With reference to 

 the note at p. 131 it seems to have been supposed that the essence of 

 this kind of immersion objective would have been rested entirely 

 upon the increase of aperture which the immersion fluid renders 

 practicable. 



A homogeneous-immersion objective does of course allow of an 

 increase in aperture as compared with a dry objective or a water- 

 immersion objective in the proportion of 1 • to 1-33 and 1 • 52 — 

 those figures iDeing the refractive indices of the three kinds of objec- 

 tives, — and the larger apertures so obtained constitute the practical 

 benefit to be derived by the investigator from the use of homogeneous- 

 immersion objectives. When, however, we speak of the " essence of 

 homogeneous immersion " it is obvious that the essence, in the strict 

 sense of the term, must relate to something which is common to all 

 kinds of homogeneous-immersion objectives — not simply to those of 

 high angles only but to low ones also, and the correct use of that 

 term is not affected by the fact that low-angled homogeneous-immer- 

 sion objectives are not found in practice. 



The essence of homogeneous-immersion depends therefore on the 

 greater facility which is afi'orded for correcting objectives of very 

 wide angles ; the practical advantage is the increase of aperture which 

 can be obtained by the use of a fluid of higher refractive index than 

 water. 



Abbe's Apparatus for demonstrating the Increase of Eadia- 

 tion in Media of higher Eefractive Index than Air.— We should 

 have added to the description of this apparatus given at p. 343 a 

 note as to the eftect of removing the plate B and observing the two 

 fields /i and f^ direct. In this case, of course, the greater brightness 

 is not seen at /\, but, on the contrary, it looks slightly less bright. 



