96 Optical Improvements \^Z^llf"w^^^' 



The pollen of Ajuga genevensis is yellow, but that of A. pyra- 

 midalis is usually white ; again, while the grains of Ornithogalum 

 umhellatum are large and yellow, those of 0. nutans are small and 

 white. 



Some objection may be raised to any reliance being placed upon 

 the di-y shrivelled-up grains of herbaria specimens — such specimens 

 being in most cases the only ones obtainable for purposes of in- 

 vestigation ; but the structure of pollen is such as to bring into 

 greater prominence the pores, folds, valves, and other markings 

 which are met with on their surface after the grains have collapsed 

 by the discharge of their contents. 



In regard to the mounting of these objects for the microscope, 

 they show to the best advantage when put up perfectly dry ; the 

 cells should be sufficiently shallow to admit of no more than a single 

 layer, and at the same time deep enough to permit the grains to 

 move about. If pollen is mounted soon after it has been discharged 

 from the fresh anthers, the fo villa is apt to condense on the covering 

 glass, and the slide soon becomes useless. The stamens taken from 

 an unopened flower-bud furnish the best and cleanest pollen, and 

 these should be selected in preference to those taken from the fully- 

 developed flower. 



Canada balsam, glycerine, and other media are occasionally 

 helpful in making out structure ; thus the pores of Campanula 

 Totundifolia, Pliyteuma Halleri, and other allied species, are made 

 much more distinct when mounted in balsam. — Paper read before 

 the Literary and Philosophical Society of Manchester. 



VII, — On Professor Listing's* recent Optical Improvements in 

 the Microscope. By De. H. Hagen. 



In aU microscopes the dioptric arrangement is now analogous to 

 the astronomic spy-glass ; they have but one real image, from 

 which the virtual image is formed and brought to the eye of the 

 observer. 



Professor Listing proposes to have two real images, and in 

 this way to form three successive augmentations instead of two, 

 as before. It is well known that by a prolongation of the draw- 

 tube, or by increasing the distance between the objective and the 

 eye-piece, the image becomes successively greater, but the defi.ni- 

 tion and penetration is by no means better. Professor Listing has 

 made some experiments, and states that with an eye-piece of his 



* See also 'Nachr. d. kgK' Gesell. der Wissensch.,' 1869, No. 1, and Poggen- 

 dorff s ' Annalen,' 1869, T. xvi., p. 407. 



