378 HOW TO SEE WITH THE MICROSCOPE. 



you have a problem in hand, to which, for exquisite intensity of 

 definition, or nicety of manipulation, the resolution of the 

 Moller or Nobert plates are child's-play matters ! And if you 

 should be desirous of testing your lenses as to their capacity for 

 histological work, the trachea will be found to be one of the 

 very best test objects in existence, and will surely furnish you 

 valuable ideas as to the relative value of wide and low angled 

 glasses for the purposes of histological research. 



Not long since one of our most skilful physicians sent me a 

 sample of urine from a child supposed to be suffering from a 

 disease of the kidneys. Jt was a perplexing case, and the med- 

 ical gentleman in charge hoped that the microscope would give 

 some light on the case ; when examined with a low power l-5th 

 of 70 Q aperture, the specimen appeared in every respect 

 healthy ; but on further examination with a wide-angled glass* 

 and with an amplification of nearly 4,000 diameters, it was- 

 found to be literally swarming with vibriones. There were bil- 

 lions upon billions' of them in every field examined, all of which 

 were totally invisible to the low-angled l-5th. 



We often hear the remark, that wide-angled glasses are just 

 the thing for the display of lined objects, surface markings,, 

 diatoms, etc., but that owing to their short focal length, and 

 limited working distance the trouble Attending the adjustment 

 of collar and in general, the difficulties pertaining to their use, 

 that they are unsuited to the purposes of the histologist ; while 

 on the contrary, low-angled glasses of greater working distance 

 requiring no skill in management, are the tools with which the 

 real work of the microscope has been, and will continue to be 

 done, and such are fondly termed good, honest and reliable 

 ' working glasses." 



1 can never listen to this line of argument without entertain- 

 ing the suspicion that sloth and inactivity lay at the bottom. We- 

 never hear astronomers complain of the care they are compelled 

 to use in instrumentation ; on the contrary, they pride them- 

 selves on the accomplishment of being able to work instrument* 

 requiring a great amount of skill and precision in manipula- 

 tion. 



The objection of short working distance originated years ago. 



