NOTES. 381 



Divide 100 by M, which will give the value of one division of 

 the eye-piece micrometer in micra. This is useful where the 

 measurement of any object is some simple number of eye-piece 

 divisions. 



With the same 1/2-inch we have 100/1175 = 8*51 micra. There- 

 fore if any object spanned two or three divisions its size is known 

 at sight. (The card of magnifying powers, which I put in the 

 box of the new binocular, has both M and 100 times its reciprocal 

 given, the tube length being 211 mm.) To find the constant 

 of the eye-piece : The combined magnifying power of this objec- 

 tive and each eye- piece must be multiplied by this quantity 8 '55 ; 

 thus for No. 3 eye-piece we have 175 x 8*51 = 1,489, the constant. 

 If this constant be divided by the value of an eye-piece division 

 in micra for any other objective it will give the combined magni- 

 fying power; thus for the l/6th object-glass where 100/ilf = 3 •Q- 

 we have 1,489 divided by 3*0 = 500 as before. 



The systems are the same, except that division is substituted 

 for multiplication and vice versa. As division is arithmetically 

 more laborious than multiplication, this system is hardly so con- 

 venient as the other, but when logarithms or a slide rule is used 

 there is no difference between them. 



CARL DIETRICH AHRENS. 



{Died March, Uth, 1918, aged 81.) 



With the death of Mr. CD. Ahrens * a notable worker in the- 

 optical and microscopical worlds has passed away. By trade he 

 was a prism and spar slitter. He made Nicol prisms and analysers^ 

 quartz and calcite prisms of all kinds, as well as the glass prisms^ 

 for Wenham binoculars. In 1867 he designed a binocular upon 

 quite a novel plan. The rays issuing from the back of the objec- 

 tive were separated to an angle of 15° by a double-image calcite 

 prism ; these rays were then crossed over by two flint prisms, to- 

 correct the chromatic dispersion. The rays used were the extra- 

 ordinary, the ordinary being diverted out of the path. The tubes- 

 were equally inclined to each other. He says that " the plan 

 may not be so good as the Wenham for low powers, but for high. 



* Ahrens was bom in Hanover in 1837, and as it had not then been* 

 ceded to Germany, he was by birth a British subject. 



