14 



NOTES ON THE EXHIBITION 



the large proportion of those for which the half-inch objective 

 answers best. Lower powers are most serviceable if there are 

 other stands — the inch for small insects. Everyone knows how 

 welcome the parasites (under the microscope) are ; also for 

 double-stained plant-sections, hairs, scales, and for rock-sections. 

 The beauties of mosses, algae, etc., are best shown with the 

 two-inch. 



Demonstrating or class microscopes are most valuable in 

 lectures on natural sciences, but their use must be generally 

 restricted to professional teaching. I have seen them used by 

 Dr. Beale many years ago at the College of Physicians, when he 

 was demonstrating his " Germinal " and " Formed " matter. In- 

 struments like his are now made by Messrs. Salt and Son, 

 of Birmingham, Fig. i. 



a drawing of 

 which is an- 

 nexed. Those 

 made by Parkes 

 and Son, of Bir- 

 mingham, were 

 represented in 

 Part I of this 

 Journal. 



But if an 

 object is to be 

 shown and ex- 

 plained to a 



number of persons at the same time, it is evident that some 

 method of projection must be employed. I find that men of 

 much experience in matters of this kind think that it is best to 

 get a picture of the object, and project that upon a screen. 



In Part 2 of this Journal, Mr. H. Barker gave a very clear 

 account of the processes for photographing magnified objects ; but 

 in order to obtain good negatives, much practice and skill are 

 recpired, and the printing of transparencies is a further difficulty ; 

 manufacturing opticians are now offering a great variety of such 

 slides. Drawings on glass roughened by emery, or a thin layer of 

 photographic varnish, are sometimes used instead of the photo- 



