A71 Improved Method of Apertometry. 345 



Section 6. — Error in Apertometry due to Chromatic 



Aberration. 



Abbe has pointed out (i) that objectives corrected for chromatic 

 aberration may show considerable error if the path of the beams 

 is altered. Thus, with high-power objectives, the aperture as 

 measured with blue rays is usually larger than that obtained with 

 red ; with the holoscopic 4 mm. the converse is the case. In 

 measuring aperture accurately, therefore, monochromatic light 

 should be used of known, and if possible of standard, wave. 

 This should be either the " chosen colour " (i.e. that for which the 

 most perfect corrections have been made —usually yellow-green 

 rays), or light of short wave length (e.g. blue raysj when the 

 maximum resolving power is required of which the objective is 

 capable. 



For providing monochromatic light for this purpose good colour 

 filters are quite sufficient. They may be placed anywhere in the 

 light path ; between the source and its aperture is a convenient 

 position. 



Section 7. — Error in Methods based on the 

 Measukement of the Eamsden Disc. 



The employment of these methods depends on the accuracy of 

 the formula N.A. x 2 D = M x R.D., where D is the distance of 

 distinct vision (10 in., or 254 mm.), M is the total magnification 

 of the instrument, and E.D. is the diameter of the Ramsden disc. 

 Now Ainslie {3), who has given a description of the method, points 

 out that for accurate determinations the diameter of the Eamsden 

 disc must be measured (because of its small size) by means of 

 instruments of considerable accuracy. When this has been done 

 it still remains to determine the magnification and to fix the value 

 of D, because the accommodation of the eye renders the value of 

 the latter uncertain. For this purpose I find the Eikometer of 

 Wright {4) to have definite advantage, because it fixes the value 

 of D (not at all necessarily at 10 in. however) during the deter- 

 minations of the magnification. Apart however from .the requii^e- 

 ment of special instruments I have found the method to suffer 

 from inaccuracy, which is probably due to irradiation. 



Section 8. — Desgiuption of an Impeoved Method 

 of Apertometry. 



The principles on which this method rests have been briefly 

 considered above ; the description will therefore be limited to 

 structural features. 



