" HOLOSCOPIC " EYE-PIECES 



379 



powers the proper position of the eye is further from the eye-lens 

 than is the case with those of the Huygheiiiaii construction ; which 

 makes it as easy to use an eye-piece of as great a power as 18 or 27 

 as one of 4 or 8. 



The field of these eye-pieces has, as we .believe, been very wisely 

 limited to five or six inches. The attempt on the part of English 

 opticians to give to our eye-pieces fields reaching eighteen inches is 

 an error. A microscopic objective with the lowest aperture has the 

 field greatly in excess of any other optical instrument ; and to deal 

 with such eccentrical pencils as must be engaged by an eye-piece 

 with a field of eighteen inches is a strain not justified by what is 

 gained. 



The powers of the working eye-pieces are also arranged in a new 

 way. The multiplying powers for the long tube are 4, 8, 12, 18, 27 ; 

 it will be seen at once, therefore, that they bear no definite ratio to 

 one another, and if we seek to simplify the focal lengths we are, by 

 the employment of the metrical system, confronted with decimal 

 fractions. But without further elaboration it may be well to say 

 that 12 is the most generally useful eye-piece, 

 and if only one compensating eye-piece is to be 

 selected, there can be no question, from a prac- % 

 tical point of view, but this is the best to em- 

 ploy. The 4 is too low, and 

 the 27 is too high for general 

 purposes, and the 8 and 18 are 

 sufficiently near the 1 2 to give 

 the latter the advantage in 

 general work. We cannot, 

 however, refrain from the ex- 

 pression of the opinion that a 

 series of 5, 10, 20, or 6, 12, 24 

 powers would be in many senses 

 more useful, and would offer 

 facilities in application not se- 

 cured by the series of Abbe now in use. 



It may be well to give further emphasis to the fact that this con- 

 struction of eye-piece is not only essential to the proper work of 

 apochromatic objectives, but they greatly enhance the images given 

 by ordinary achromatic lenses ; and it may be noted that the 8, 12, 

 and 18 eye-pieces for the short tube are identical with 12, 18, 27 

 for the long tube. The 4 eye-piece for the short tube makes a very 

 suitable 6 power for the long tube. 



A new series of eye-pieces has been recently introduced by 

 AY. Watson and Sons, to which they have given the trade name of 

 ' Holoscopic.' What is held to be a very simple method is employed 

 for rendering them either over- or under-corrected, and therefore 

 suitable for either apochromatic or the ordinary achromatic objectives. 



This eye-piece is of the Huyghenian type, but unlike the ordinary 

 pattern the eye-lens, together with the diaphragm, is mounted in a 

 tube which slides telescopically in the body of the eye-piece, at the 

 lower end of which the field-lens is fixed. This is shown in fig. 327. 

 When the sliding tube is pushed home as far as it will go, the eye- 



Fio. 826. Abbe's 

 comp ensating 

 eye-piece of 12 

 power. 



FIG. 327. Watson's 

 holoscopic eye- 

 piece. 



