APPENDICES 



149 



possible, it is necessary to employ an i8-compensating eye-piece. As this is a positive one the 

 threads can be focussed just in the same fashion as with the ordinary Ramsden eye- piece, but the 

 micrometer requires a special fitting, and, as the lenses are in such close proximity to the webs, 

 great care must be taken they do not touch them. It is needless to state if this accident occurs they 

 are nearly sure to be broken. 



APPENDIX III 



TO OBTAIN THE NUMBER OF LINES TO THE INCH IN A SPECIMEN 



BY PHOTOGRAPHIC METHOD 



Another method for obtaining the number of lines to the inch is by the photographic method. 

 A negative is taken of the specimen, and then of a stage micrometer ruled to ten-thousandths, which 

 is substituted. Great care must be exercised that the length of the drawtube is not touched during the 

 process. A comparison between the two negatives is then readily made. This method is the most 

 exact possible. 



APPENDIX ly 



TO DETERMINE RATE OF PERIODIC STRUCTURES TO INCH OR MM. 

 IN A SPECIMEN WHEN AMPLIFICATION IS KNOWN 



It is often required to determine at what rate to the inch, or to the mm. periodic structures, 

 occur in the photograph of a specimen when the amplification is accurately knoion. 



Example. — An object is known to be magnified looo diameters, and it is found i8 dots can be 

 counted in -3 of an inch when the photograph is examined. The question is, at what rate per inch 

 is the structure ? 



mag. power x the number counted, 

 space counted 



thus 1000 X 18 ^ . , 

 = 60,000 per inch. 



•3 



If the answer is required in mm., seeing that 25*4 millimetres equal an inch, the amount in inches 

 must be divided by 25*4 (Carpenter). 



; APPENDIX V 



HOW TO USE A MILLIMETRE MEASURE FOR ASCERTAINING THE 

 SPACE IN WHICH A NUMBER OF PERIODIC STRUCTURES 

 CAN BE COUNTED TO THE INCH 



Suppose a millimetre measure be used to ascertain the space in which the number of periodic 

 structures in the photo-micrograph are counted, and the rsite per inch is required, the method is as 

 follows : 



Example. — If, with a magnification of 675-2 diameter, 12 dots can be counted in 7 mm., then 

 because i mm. = "03937 inch, 



675 2x12 _ 2Q 400 per inch. (Carpenter.) 

 7 X -03937 r 



