98 J. E. INGPEN ON A PROPOSED STANDARD DYNAMOMETER. 



be fixed, low powers are registered as magnifying less than tliey do 

 in practice. The number of divisions on the scale B corresponding 

 to one or more on the scale E, gives at once the power of the 

 objective. Thus, if ten divisions on B cover one on E, the power 

 of the objective is ten diameters, and should be registered x 10. 



With objectives having adjustment for thickness of covering 

 glass, and those with wet and dry fronts, two or more measures 

 may be taken. One of the micrometers supplied is uncovered, and 

 can have glass covers of known thickness laid on it : thus, the 

 power of an objective can be found at " uncovered," at " covered" 

 for a known thickness of glass, or at " covered " with a known 

 position of the divisions on the screw collar, or with a wet front 

 under any of these conditions, thus affording a complete registra- 

 tion of the power of the objective. 



The instrument is constructed so as to be complete in itself; the 

 rack at F being sufficiently delicate for focussing the scale E, with 

 due care, even on the highest power objectives, but it would be 

 better for the tube A to be attached to a firm microscope stand, 

 with a good slow motion. The tube D can then be dispensed with, 

 and the scale E taken out and viewed, as an ordinary object, on the 

 stage of the microscope. 



There is no difficulty in reproducing such an instrument as this, 

 bearing in mind the one point essential to it as a standard, viz., 

 that the front of the objective must be exactly ten inches from the 

 upper or eyepiece micrometer. The value of the scales, whether 

 French or English, &c., is of no consequence, provided they are 

 equal or one a known aliquot of the other. The scales should be 

 carefully tested, by reversing their positions or other means, as, of 

 course, much depends upon their accuracy. 



The distance of ten inches has been selected, as about that most 

 usual in England. It seems that in America longer, and in Fi:ance 

 and Germany, shorter, bodies are preferred. The distance of 250 

 millimetres (9f inches) has been suggested, which would not make 

 much difference, but it would probably be best to leave the Quekett 

 standard at ten inches, and not to alter it without good cause. 



It is hoped that this standard will render unnecessary many of 

 the calculations which are so troublesome and unsatisfactory, such 

 as attempting to find focal centres of compound systems, com- 

 parison with a one-inch lens (which would be probably only correct 

 for parallel rays), &c., &c., while the mode of operation is, it is 



