THE PAST AND FUTURE OF THE MICROSCOPE 207 



ohjectiv^es should be manufactured at a lower cost than the 

 apochromatics; and the photographic objective for blue or 

 \iolet light would perhaps be especially useful. For 

 photography is one point in which microscopy appears 

 most backward, as compared with astronomy. 



Medium-power Microscope. — The writer has already 

 suggested a medium-power microscope, with erect image 

 and parallel tubes, made by combining the Swan and Porro 

 prisms. It is possible that it could be made with only 

 four reflections on each side. 



Water Immersion. — In the future it is to be hoped that 

 the water-immersion objective, which was driven out, 

 for a time after 1878, by the oil-immersion objective, so 

 successfully used by the bacteriologists, would come again 

 into use for the examination of structures and organisms 

 of thickness exceeding a few microns in water and watery 

 fluids. Here it is superior, if skilfully used, to the oil- 

 immersion objective of the same aperture. A water- 

 immersion achromatic condenser calculated for standard 

 slides is also a desideratum. 



Monobromide of Naphthalin Immersion Objective. — 

 This can be used without a cover-glass (with a 30-milli- 

 meter collar) on well-stained (or unstained) thin smear 

 preparations of bacteria, or pressed-out early stages of 

 chromosomes in the reduction division. 



Photography. — It may also be hoped that the future will 

 see a greater use of photography, the specimens being 

 specially prepared and stained with a view to photograph- 

 ing them and working on (measuring) the photographs, 

 as is done in astronomy. Books on minute anatomy or 

 cytology should certainly be illustrated with abundant 

 good photographs — and would be, were such available 

 (as they are in bacteriology). 



Light. — As has been already noted, the use of ultra- 

 violet light for photographs might be extended by some 

 of the biological institutes, which can afford the necessary 

 expenditure of money and time. Illustrations produced 

 by the objective of 2.5 equivalent aperture, might, one 

 would think, be plentiful in the future. 



