ZOOLOaT AND BOTANY, MICKOSCOPT, ETC. 771 



need to be followed by every one who would use the instrument, but to 

 be acquainted with the main facts of Abbe's theory — to comprehend 

 the doctrines he has propounded and the experiments by which he has 

 made it plain, so as to use it in the interpretation of what the lens 

 reveals, is as necessary for the one who would be a well-skilled 

 observer as for him who would improve the powers of the instrument 

 itself. 



The best natural endowments of clear vision and delicate touch, 

 and the greatest attainment of that 'manual dexterity,' which, as 

 Beale says, ' although subordinate to many higher mental qualifica- 

 tions, is essential for the successful prosecution of microscopic 

 observation,' are not enough, unless guided by that clear mental per- 

 ception of the general principles of optical physics which can help the 

 eye to recognize the origin of the appearances it sees and lead the way 

 to decisive experiment. The studies of Abbe in particular have done 

 more to establish a firm footing for further improvement of the Micros 

 scope and a more intelligent use of it in the form we now have, than 

 all the laborious but ill-directed efforts of a host of other workers. 

 As a knowledge of chemical science has led to a great advance in the 

 use of reagents, mounting media, hardening, clearing, and other 

 preparatory fluids, so a knowledge of the laws of light is essential to 

 the proper use of the Microscope in examining the objects prepared. 

 To discriminate between bubbles of air or globules of oil in 

 water, to understand what forms a transparent, solid, or hollow 

 cylinder may appear to take by transmitted or reflected light, and in 

 media of an index more or less varying from its own, have long been 

 recognized as questions the microscopist should exercise himself upon 

 by theory and practice till he cannot be misled. Yet how often still 

 are men misled in these cases ? Especially important is it to learn to 

 discriminate between proper and imperfect focusing, and to use the 

 adjustment collar of the higher power lenses to the best effect, 

 ^ There is no doubt,' says President Duncan, of the Eoyal Society, 

 ' that, with very few exceptions, the microscopic work relating to the 

 morphology of the animal and vegetable kingdom has been conducted 

 either without corrected objectives or with those which have an 

 average adjustment,' and, remarking that very minute bodies appear 

 abnormally thick from lack of correction, &c., when highly magnified, 

 goes on to say he has no doubt but that similar abnormalities are 

 constantly recorded as truths. So, too, there is no doubt that lines, 

 fine dots, and beaded structures of various kinds have been constantly 

 misunderstood. Lines have been recorded which have no real exis- 

 tence, or which, if existent, are neither so wide nor so numerous as 

 they appear to be, nor in the direction they appear to lie. A careless 

 use of the diaphragm, a more or less complete employment of the 

 aperture of the objective, or of one part of that aperture rather than 

 another, or error in focusing, may transform elevations into depres- 

 sions, squares or triangles into circles, or rhomboids, or hexagons, or 

 simple lines, and vice versa. One of the most interesting questions 

 we are called to meet to-day, as it seems to me, is whether we can 

 discover any sui:e and satisfactory diagnosis of the real nature of 



