Notes on some Focometric Apparatus. By F. J. Cheshire. 515 



but a disk D in which is cut a narrow slit with its length 

 parallel to the scale divisions, as indicated to the left of Fig. 53. 

 With this device the area of the slit determines the amount of 

 light passing through the lens from each point of the scale S, the 

 length of the slit determines the effective focusing aperture of the 

 imaging pencils for the horizontal line, in a plane at right angles 

 to it, whilst the width of the slit only is effective in a plane at 

 right angles to the direction of the scale divisions. The adjustment 

 of the eye-piece scale in the plane E, can thus be effected by focus- 

 ing up the horizontal line of the scale S, with what is for this 

 purpose the full aperture of the lens, whilst the telecentric adjust- 

 ment for the scale divisions is still retained. 



Figs. 52 and 53 may be taken to illustrate the action of a slit 

 telecentric disk by supposing that in the former figure the horizontal 

 auxiliary focusing line referred to, and the length of the slit, are 

 respectively normal to, and in the plane of the paper. In Fig. 53 

 both the scale divisions and the length of the slit must be con- 

 sidered as normal to the paper. 



Substage Focometer. —Mr. Blakesley in his "Geometrical Optics" 

 (1903), in discussing various methods for finding short focal lengths 

 " such as those of eye-pieces and object glasses of Microscopes," 

 states that, " a collimator of about 3 cm. focal length and with a 

 small hole or two fine parallel lines in its principal focus as an 

 object may be mounted in a hole of the stage of a Microscope. Any 

 lens to be experimented upon is then placed over this arrangement 

 and the final image measured." After considerable experience 1 

 have come to the conclusion that when certain precautions are 

 taken, which I shall proceed to explain, this method is the best and 

 most convenient one for the needs of the ordinary mici'oscopist. 

 In the form which I prefer the collimator consists of an achromatic 

 Jens system, with an equivalent focal length of 26 mm. mounted 

 at the middle of a short tube, which is fitted at its lower end, in 

 the plane of the lower principal focus of the lens, with a millimetre 

 scale, and at its upper end, in the plane of the upper focus of the 

 lens, with a metal disk D in which a narrow diametral slit (1 mm. 

 wide) is cut to secure telecentric adjustment in the way already 

 described. This collimator is slipped into the condenser ring so 

 that the slotted disk is practically flush with the surface of the 

 stage. The lens to be tested, if an objective, is screwed into the 

 nose of the Microscope body and focused on the telecentric slit, 

 when upon looking down the tube an image of the collimator scale 

 will be seen in the upper principal focal plane of the objective. To 

 measure this image it is only necessary to fit up the draw -tube as 

 an auxiliary Microscope by screwing a low-power objective into its 

 lower end and fitting its upper end with an eye-piece, carrying a 

 micrometer scale — conveniently, 1 cm. divided into a hundred 

 parts. A second image of the collimator scale is projected on to 



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