61 



and the magnifying power could be immediately found by a table of reciprocals, 

 and a simple multiplication. Another plan, requiring no calculation, is to 

 obtain an eye-piece micrometer so ruled that when placed in the eye-piece the 

 same number of divisions fill the field of view as there are tenths of an inch in 

 that field when drawn at 10 inches, as above described. In using this arrange- 

 ment, multiply by 10 the number of eye-piece divisions that cover one hundredth 

 of an inch of the stage micrometer, and the result obtained is the magnifying 

 power. Thus, if 41 eye-piece divisions exactly cover one hundredth of the stage ' 

 micrometer, the magnifying power would be shown to be 410. This method 

 requires a separate scale for each eye-piece employed. 



The Chairman thought that the scale was a very simple contrivance. 



Mr. B. D, Jackson inquired whether Dr. Matthews used an absolute tenth of 

 an inch, or whether he took steps to counteract the magnifying power of the 

 eye-piece. 



Dr. Matthews said that the advantage of his plan over that just proposed was 

 that it could be applied to any eye-piece, and was not adapted merely to the 

 one for which it, was made. The ^q in. was increased by the eye-piece, and 

 varied with the power of the eye-piece, but he was not bound to adopt this par- 

 ticular distance ; any other known distance would do equally well, and as the 

 value of the right hand screw was 50 threads to the inch, and the head was 

 divided oflfinto 10, he could adjust the points with great accuracy. 



Mr. Ackland observed that inasmuch as the space employed by Dr. Matthews 

 was a portion only of the field of view, any error which might occur would be 

 increased in proportion to the ratio of the space between the points to that of 

 the diameter of the field. 



Mr. S. J. Mclntire said that he had for a long time adopted a plan similar to 

 that suggested by Mr. Ackland, but had done so in what he thought was a 

 more simple way, without the use of any tables or complex formulae. His 

 method of procedure was as follows :— First ascertain accurately the apparent 

 diameter of the field of the eye-piece, and reduce it to thousandths of an inch. 

 Next place the micrometer 6n the stage and read olF the number of divisions 

 which measure the diameter of the field, reducing them also to thousandths • 

 then divide the number of thousandths in the apparent diameter, by the number 

 seen on the stage micrometer, and the quotient will be the amplification re- 

 quired. 



JS.g., if the field apparently measures 5 in., and -[gg^j in, are seen on the 



stage micrometer, the sum will be 



5000 

 ^=200 



or should the field be 7^ in. and ^5^0 in. be seen on the micrometer, it would be 



Z^OO ^ 300 



the amplification in these two cases being thus 200 and 300 diameters re- 

 spectively. 



The Chairman thought that Mr. Mclntire's plan was a very simple one, and 

 one which could be followed by anyone. 



The Secretary announced that Mr. Eichards had brought for exhibition a tube 

 with a glass end, which was placed over the objective of his microscope for 

 observing objects under water. 



Mr. Richards said that this arrangement had been found very useful in con- 

 nection with his erector for dissecting objects under water. 



