106 THE MICROSCOPE. July 



the power of the converging lens, is the power of the 

 diverging lens. Its focal length can then be determined 

 in either inches or mm., by inspection. Example : The 

 power of a double concave, when combined with a bi-con- 

 vex, is 0.85, as measured on the P side of the rule. The 

 power of the bi-convex alone is 3.0 ; then 0.85, minus 3.0, 

 equals minus 2.15. The focal length of the double con- 

 cave is therefore minus 4,65 inches or 118 mm., these 

 being the figures in a line with 2.15 on the rod. 



(4.) The rule is very useful as a ready reckoner. Ex- 

 ample (A) : A lens of 8i inches focus is combined with 

 one of 94 mm. focus — required the power and focus of 

 the combination in inches and mm., Hh inches is in a 

 line with 1.175 P, and 94 mm., in a line with 2.7 P. 

 The power of the combination is, therefore, 1,175, plus 

 2.7, equals 3.875 ; this is in a line with 2.58 inches and 

 65.3 mm., the foci required. Example (b): I have a 

 lens 178 mm. focus — what must be the focal length of 

 the lens in inches that added to it will yield a power of 

 5? In a line with 178 mm., is 1.425 P; then five minus 

 1.425 equals 3.575, the power of the lens required; this 

 is in a line with 2-8 inches, which is its focal length. — 

 Quekett Microscopical Club Journal, 



Reduction In Sulphates. — Spirillum desulfuricaus is the 

 name of an org-anism obtained by Beyerinck from ditch 

 water which has the property of reducing- sulphates. It 

 is a short spirillum, actively motile, but its motion ceases 

 when oxyg-en is applied. — Arch. Neerland. 



Does Tuberculin Accelerate Tuberculosis in Animals.— 

 Prof. Nocard, the French veterinarian says : "The ag-- 

 gravation of tuberculosis lesions under the influence of 

 tuberculin is a common fact in man, it is exceptional in bo- 

 vines. I have observed but three cases out of 3500 injec- 

 tions that I made myself, and besides, when it takes place 

 it is always upon animals with phthisis to the last deg-ree 

 and consequently useless." — American Veterinary Review. 



