304 



MEMORANDA. 



meter which measures one division in tlie stage micrometer, 

 and let D be the distance between the two micrometers. 



Then ¥, the equivalent focal lengthy is equal to -vf^Tv 



This rule was new to me, and may be so to other readers 

 of the Joiu-nal. I have, therefore, sent you the results ob- 

 tained by applying it to my instrument, adding, for the sake 

 of comparison, and to show the correctness of the rule, the 

 power of each glass Vrith the A eye-pieces, as obtained with 

 the camera lucida a long time ago. 



If there were no cause of error, the right hand column, 

 divided by that next to it should give, in every case, the same 

 quotient, viz., the power of the eye-piece. But the slight 

 discrepancies that will be found are readily accounted for by 

 the impossibility of obtaining exact measurements with a 

 micrometer eye-piece of this description, particularly when the 

 lines on the stage micrometer are magnified with high powers. 

 Besides which the draw -tube was sometimes pulled out a 

 little, which would obviously make a difference, i.e., make 

 the powers obtained in this way differ from those obtained 

 by the camera lucida, when the draw-tube was not moved. 



Focal length, &c., of a set of objectives obtained by the 



rule, r= 



N + 1 



2-inch 



