156 A^nerican Quarterly Microscopical Jourjial. 



out the spectral images and admit the dioptric beam, the 

 grosser parts of the object, the outlines e. g., will be visible, 

 but not a trace of the minute structure. Mr. Crisp was pro- 

 vided with a series of diaphragms placed behind the objective, 

 which enabled him to illustrate these facts. 



It is well known that the finer the ruling the greater is the 

 dispersion produced, hence, our diffraction images will be 

 more or less widely separated according to the closeness of 

 the lines on the object. By taking two series of lines, one set 

 ruled twice as closely as the other, the diffraction images of 

 the latter will be twice as far apart as those of the former. It 

 is a curious fact that by arranging a diaphragm to cut out the 

 intermediate spectra of the coarser band (thus leaving spec- 

 tra corresponding in position to those of the finer set of lines), 

 we will see in the microscope double the number of lines act- 

 ually present; /. e., the same number as are in the finer band. 

 We have not space to treat this subject more fully now, but 

 we hope to give a good account of Prof. Abbe's investiga- 

 tions in the near future. It appears that the image in the 

 microscope is of a peculiar spectral nature, and does not neces- 

 sarily show us the true structure of an object; or, as Mr, 

 Crisp says : " It is not possible to determine the real structure 

 of Z'. angulatum merely from its image as presented by the 

 microscope." It would seem that the microscopist of the 

 future will need to be also a mathematician, if he is to study 

 minute structure. 



YELLOW FEVER. 



Medical literature has lately embraced many contributions 

 relating to yellow fever and the germ theory of disease. 



In the American Practitioner of November, 1878, Dr. J. B. 

 Marvin, of Louisville, has an article upon yellow fever which 

 is of sterling value. The microscopical observations may be 

 stated briefly as follows: "Pure glycerin was placed upon a 

 slide and breathed upon by the patient. After a few minutes, 

 large quantities of active vibrios and bacteria were revealed. 

 The blood-corpuscles were crenated, frequently breaking up. 

 After the occurrence of black-vomit there was found a great 

 increase in the number of white corpuscles, frequently one 

 white to five red ones. Bacteria and vibrios were found in the 

 blood, as many as five or six being found in the field at once. 



