242 E. M. NELSON ON A NEW METHOD OF MEASURING THE 



direct projection on to a scale, without the intervention of any 

 camera lucida or drawing instrument ; the position of the 

 Ramsden's disc, from which the 10-in. projection distance is 

 measured, is easily found by means of a piece of ground glass. 

 An excellent scale for the measurement of low powers is a 

 Lufkin 3-in., No. 2111, price Is. 



[Practical members ma} 7 , by this time, be ready to ask, " What 

 is the practical use of this system ? ' As a general answer it 

 might be said that it shows how the materials for an important 

 microscopical measuring tool can be easily determined. 



But another practical reason for my taking interest in our 

 veteran member's paper is the hope that it will stimulate some 

 of us to take an increased interest in microscopical measurements. 



I hardly need to impress on members the value of actually 

 measuring objects, beyond offering a reminder, that measurements 

 are the fundamental basis of microscopical science, and of every 

 branch of science. Some would, perhaps, claim to put mathe- 

 matics in this honourable position, but mathematics would be in 

 a most sorry plight without measurements in various forms. 



Mr. Nelson, in a letter, says : " The combined magnifying 

 power is wanted for drawings. Beale's method of exhibiting a 

 drawing of the stage micrometer with the picture is quite the 

 best, but it is adopted by only a few authors." And he mentions 

 an instance of great trouble being caused by some drawings in 

 books on microscopical subjects having the magnifications wrongly 

 stated in the legend. 



It will have been noticed that Mr. Nelson has, hitherto, con- 

 fined the use of the "eyepiece constant," for one eyepiece, to one 

 definite tube length for one constant ; but used it for getting 

 the total magnification with varying powers of objectives. 



Tests have, however, shown that the total magnification can be 

 determined, by his method, for different tube lengths just in the 

 same way as for different powers of objectives. Mr. Nelson 

 himself says that " increase of tube length increases the power 

 and, of course, diminishes the field, and is just the same as 

 putting a higher-power objective on the nosepiece ; the constant 

 of the eyepiece remains the same." In support of this statement, 

 I give below r a few of the results of experiments made by Mr. 

 Nelson not many days ago. 



