OP ARTS AND SCIENCES. 143 



thousand diameters, with wonderful results, which fully justify me in 

 saying all that I have in regard to the study of thick tissues with low 

 powers having Avide angles of aperture,* 



" I will take a young fish as an example to illustrate the remarkable 

 efficiency of the flat field. In a view from above, one may see no less 

 than six or seven different layers or sets of organs resting one over 

 the other ; first the skin and the muscular layer, next the vertebra3, 

 within these the spinal marrow, and below the latter the chorda dor- 

 salis, and close to this the dorsal artery, then the intestines and their 

 appendages ; and yet every one of these may be plunged through and 

 totally ignored, on account of the peculiar properties of the flat field, 

 and the last, the intestines, minutely inspected, not only cell by cell, 

 but each cell may be studied, in every particular of detail, as if it were 

 isolated. And so may any set of organs be treated, whether situated 

 above or below in the animal. With such means at hand, as long as 

 cells may be seen with a very moderate light, it is utterly preposterous 

 to trust what may be worked out by separating these organs from the 

 animal, piecemeal. When intact, every cell may be measured, not 

 only transversely, but also with the greatest nicety in a perpendicular 

 direction, by the micrometer screw, which works the fine adjustment of 

 the objective ; every cell, indeed, may be treated as if it were a sepa- 

 rate body ; but who would warrant to measure, for instance, the size of 

 the cells of a nerve after it had been removed from its natural position, 

 and with more or less inevitable distortion ? Unfortunately, investiga- 

 tors have been compelled to do this too often, up to this very day ; but 

 now I hope for much better and more trustworthy results. 



" In Embryology, how beautifully this almost transcendental defini- 

 tion of the objective applies ! All the cells of an embryo of a certain 

 age may be represented by a circle, with a smaller circle Avithin known 

 as the mesoblaat (nucleus). At successively later ages we find the 



* In this connection I would urge upon students the necessity of avoiding the 

 higher powers of the microscope in the commencement of their studies. "When 

 they have learned to use the lower objectives, it will be a much easier matter to 

 master the higher ones. Students usually suppose that they can see everything 

 with the higher powers, whereas they are greatly mistaken ; as much as one would 

 be who should make a minute inspection of the stones of some great architectural 

 pile, and then think he had obtained a proper conception of its magnificent plan 

 and glorious proportions. 



