52 



THE AMERICAN MONTHLY 



[March, 



I will ask if it is reasonable to 

 suppose that a structure that can be 

 seen " with great ease " could have 

 been overlooked by such men as Beale, 

 Balfour, Carpenter, Frey, Biitschli, 

 and a host of, other equally compe- 

 tent observers, and reserved for an 

 individual of to-day to discover. Yet 

 this is what Dr. Heitzmann declares 

 he has discovered, and upon this dis- 

 covery he has built up a totally new 

 and comprehensive theory of the 

 structure, not only of living matter, 

 but of the whole living world, con- 

 necting all the different tissues of the 

 animal and vegetable body through 

 this reticulum, and utterly discard- 

 ing the cell-doctrine, which has ren- 

 dered such excellent service to science 

 for thirty years. To an experienced 

 microscopist, the idea seems prepos- 

 terous. The objects most familiar to 

 the histologist and pathologist, com- 

 ing almost daily under the eye of the 

 physician who uses the microscope in 

 in his practice, are now declared to 

 possess a distinctly reticulate struct- 

 ure, never before even dreamed of. 



Another convincing demonstration 

 of the glaring inaccuracies of the ob- 

 servations of these gentlemen has 

 been well made by Dr. L. Curtis in 

 his " Reply to Dr. Heitzmann," pub- 

 lished last year. (Abstract read.) 



Testing Microscope Objectives. * 



BY DR. ALLEN Y. MOORE, 



With all the microscopical litera- 

 ture we have, it is surprising to see 

 how litde attention objectives have 

 received, and, at the same time nearly 

 every work on the subject tells 

 us that the objective is the most im-. 

 portant part of the entire instrument. 

 Th.it this is a fact, I think no person 

 will deny, and if the objective be 

 otherwise than perfect, a knowledge 

 of its imperfections will help the ob- 

 server by giving him an idea of how 



*Kead l^efore the Griffith Club of Micros- 

 copy, of Detroit, February 6th, 1S83. 



far his instrument may be considered 

 trustworthy. 



All first-class objectives in which 

 the angular aperture exceeds 75° or 

 80° in air, should have a correcting 

 collar; and although it makes less 

 difference with the lower powers, the 

 higher ones should be so constructed 

 that the collar moves the back sys- 

 tems of lenses. 



Bad centering and bad form are 

 conditions which are inexcusable. By 

 bad centering is meant, that the cen- 

 tres of the lenses are not in a straight 

 line. By bad form is meant that the 

 curves of the lenses are not segments 

 of true spheres. Either of these con- 

 ditions may be detected by what is 

 known as the artificial star. This is 

 nothing more than a very small glob- 

 ule of mercury upon which a strong 

 light is thrown. 



If such a globule be placed in the 

 centre of the field and carefully 

 focussed, a bright spot, which is the 

 image of the source of light, will be 

 seen. If the tube be now racked up 

 or down, so as to throw the globule 

 considerably out of focus, it will be 

 seen that the bright spot expands and 

 becomes a large circle of light. If 

 the lenses be truly formed and cor- 

 rectly centered the edge of this cir- 

 cle, or expaiision of Hglit, will be true 

 and even, but in case the lenses are • 

 not in a straight line, the circle of 

 light, or coma as it is more frequently 

 called, will not be perfectly round but 

 will seem to bulge more on one side 

 than on the other. Bad form is 

 shown by an irregularity of the out- 

 line of the coma, and the shape of 

 the coma will indicate the shape of 

 the curved surfaces of the lenses. 



Chromatic and spherical aberra- 

 tion next deserve attention. Chro- 

 matic aberration is that condition in 

 which the white light, after having 

 been decomposed by the refracting 

 surfaces of the crown lenses, is not 

 properly recom posed by the proper 

 action of the flints. When an objec- 

 tive brings the violet light to a focus 

 before the red, it is said to be chrO' 





