A New Formula for a Microscope Object-glass. 157 



to the brain itself. Thus, I think, we are in a position to trace very 

 completely the course of sounds or vibrations from a musical instru- 

 ment, or any other source, to the brain, through the medium of the 

 ear. First, the vibrations are caught and collected by the auricle, 

 and transmitted through the external meatus to the drum of the 

 ear, next across the middle ear (the tympanic) cavity, principally by 

 means of the chain of little bones, to the internal ear. Here the 

 sound is appreciated merely as a sound by the vestibular portion of 

 the labyrinth ; the direction of the sound is probably discovered by 

 means of the semicircular canals, but to distinguish the note of the 

 sound it must pass on to the cochlea. The vibration then passes 

 through the fluid of the cochlea, and probably strikes the lamina 

 spiralis, which, acting as a sounding board, intensifies and transmits 

 the vibration to the system of rods. There is, doubtless, a rod not 

 only for each tone, or semitone, but even for much more minute 

 subdivisions of the same, so that every sound causes its own parti- 

 cular rod to vibrate. Thus each string sounded on the primary 

 musical instrument induces a vibration in the corresponding rod of 

 the secondary musical instrument (the cochlea). And this rod 

 vibrating so affects the nerve cells in connection with it as to cause 

 them to send a nerve current through the nerve fibres to the brain, 

 which current is no doubt modified or affected by passing through 

 the ganglion cells, situated in the bony lamina near its junction 

 with the modiolus, as before mentioned. 



IV. — A New Formula for a Microscope Object-glass. 



By F. H. Wenham. 



A pencil of rays exceeding an angle of 40° from a luminous point 

 cannot be secured with less than three superposed lenses of increas- 

 ing focus and diameter, by the use of which combination rays 

 beyond this angle are transmitted with successive refractions in 

 their course, towards the posterior conjugate focus: until quite 

 recently, each of these separate lenses has been partly achromatized 

 by its own concave lens of flint glass, the surfaces in contact with 

 the crown glass being of the same radius, united with Canada 

 balsam ; the front lens has been made a triple, the middle a double, 

 and the back again a triple achromatic. This combination, there- 

 fore, consists of eight lenses, and the rays in their passage are sub- 

 ject to errors arising from sixteen surfaces of glass. 



In the new form there are but ten surfaces, and only one con- 

 cave lens of dense flint is employed for correcting four convex 

 lenses of crown glass: as this might at first sight be considered 



