438 SCIENTIFIC EXPERIMENTAL APPARATUS. 



8. Tangent compass. — Diameter of circle, 13.5 inches. 



9. Original Siemens'' unit. 



10. Helmholtz apparatus for vocal sounds. 



11. Koenig''s phonautograph. 



12. Koenig^s cylinder for determining the vibrations of tuning-forks. 



13. JSfacJiefs large inverted microscope. 



14. Koenig^s monocJiord. 



15. Lissajou''s comparator. 



16. Comparator for the standard yard. 



Johns Hopkins University. — Physical Laboratory, Baltimore, 



Md. 



(In charge of Professor Henry A. Rowland.) 

 The list does not include apparatus for demonstration. 



ACOUSTICS. 



All the ordinary apparatus by Koenig, of Paris, including Helmholtz's 

 double sirens, Lissajou's vibrating microscope, Hastings's pendulum com- 

 parator, «Scc. 



OPTICS. 



Meyerstein spectrometer — large model. The circle is of 16 centimetres 

 radius, divided on silver to 6' and reading by two microscopes to 2". The 

 probable and periodic errors of graduation have been investigated, and 

 are given in the American Journal of Science^ Vol. XV., p. 270. Having 

 a common axis with the large circle is a table rotating independently, 

 C.5 centimetres radius, graduated on silver limb and by two verniers to 

 single minutes. The massive stand has a joint by which the circle may 

 be brought into a vertical plane. Aperture of telescope and collima- 

 tor, 4.0 centimeters ; focal length, 34 centimetres ; powers, from 13 up- 

 ward. A smaller telescope, 2.0 centimetres aperture, 18.0 centimetres 

 focal length, i^ower 7, may be placed on a third support rigidly connected 

 with the microscope bearers. The accessories of this instrument are : 



{a.) Two telescopes with Nicol prisms before objectives ; longer diag- 

 onals of prisms, 2.0 centimetres ; length of telescopes, 20.0 centimetres ; 

 power 3 ; position-angles of prisms read by circles of 3.5 centimeters 

 radius to minutes of arc. 



(&.) Babinet's compensator with wedge of 3 centimetres available 

 length. 



(c.) High power collimating ocular. 



{d.) Low power collimating ocular. 



(e.) Three micrometer eye-pieces. 



2. Spectrometer^ by Schmidt and Haensch. The circle has a silver 

 limb 16 centimetres radius, divided to 6' and read by microscopes to 2". 



