OBJECT GLASSES AND EYEPIECES 83 



object glasses has, therefore, been limited to a somewhat more 

 restricted number of sizes than was customary some years ago, 

 with considerable advantage, as concentration on the smaller 

 number of lenses has tended to improve their quality. 



There are two types of object glasses, achromatic and Achromatic 

 apochromatic. Both types are excellent, and although there gfaS/ 

 is no doubt that the apochromatic series possess qualities which 

 render them of greater service where the most exacting scientific 

 investigation is being carried out, for the more general 

 work this extremely high quahty of optical construction is not 

 required. Hence the achromatic series fill the requirement for 

 most purposes, and are in more universal use, on account of the 

 fact that they can be made to a simpler formula and with a less 

 number of component lenses and less expensive materials. The 

 resolution of the achromatic lenses is of a very high order. As 

 an example of their good performance, the diatom " Pleuro- 

 sigma Angulatum " has dots in its structure which are approxi- 

 mately 1/48000 of an inch apart. The theoretical aperture which 

 will show these as separate dots is -5 N.A. This can be done with 

 a Beck 8-mm. achromatic object glass which has this aperture 

 (•5 N.A.), showing that an ordinary achromatic object glass, if 

 properly constructed and adjusted, is so perfectly corrected for its 

 zonal and other aberrations that it will resolve up to its theoreti- 

 cal limit. On a Grayson's ruling this lens will resolve 45,000 

 lines to the inch with a green screen and 50,000 with a blue screen. 

 For visual purposes with a colour screen, achromatic lenses can 

 be made almost optically perfect, but the apochromatic series 

 described later are more perfect as regards their colour correc- 

 tion, are better for photography, have somewhat larger aperture, 

 and will, therefore, stand the use of higher eyepieces, giving a 

 slightly better defined image, especially when a colour screen is 

 not employed. 



The chief feature of the apochromatic series is that different Apochro- 

 glasses are employed and other materials substituted, and that this passes! ^^'^ 

 combined with a different formula, involving the use of a large 

 number of component lenses, produces an object glass in which 

 there is more perfect correction for chromatic aberration. In the 

 achromatic object glasses the correction is made for two colours 

 of the spectrum, but in apochromatic lenses this correction is 

 made for three colours. For very fine markings undoubtedly 

 apochromatic object glasses give superior results ; the difference is 

 slight, but the perfection of the colour correction enables certain 

 objects to be seen with a greater crispness than is possible with 

 achromatic lenses. Although the achromatic are suitable for 

 photographic work, the apochromatic series has an advantage. 

 For those interested in the optical construction of these lenses, 

 we append a somewhat technical note on the theory of their 

 construction. 



