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This is a combination of lenses placed at the lower end of the 

 tube, next to the object examined ; its use is to define the image of 

 the object, ultimate magnification of the aerial image being effected 

 by the eyepiece. The Royal Microscopical Society have adopted a 

 standard size for screw fittings, and the intending purchaser should 

 ascertain whether the instrument he selects is constructed according 

 to that standard. Great improvements have been made of late years 

 in the construction of the objective, and as a result the apochromatic 

 lenses of to-day have reached a high degree of perfection. Objec- 

 tives are of two kinds, achromatic and apochromatic. Achromatics, 

 so called, are imperfectly corrected for colour. Apochromatics are 

 all but perfect in this respect, besides possessing certain qualities 

 which render them indispensable for critical work, especially with 

 high powers. Achromatic, or better still the semi-apochromatic, 

 lenses of some of our best makers may be relied on to perform ex- 

 cellent work, and are, moreover, of a moderate price compared with 

 the apochromatics. The latter, in the case of the -iVth inch, for 

 example, are almost of a prohibitive price, one of these by Zeiss, the 

 well-known maker, costing from ^15 to ^20. The worker may, 

 however, be assured that with a good achromatic by certain well- 

 known makers work may be done comparing favourably with that of 

 the apochromatics. The selection of an objective is an important 

 matter, and one where the advice of a competent friend is desirable. 



The essentials of a good objective are — flatness of field, penetra- 

 tion or depth of focus, definition, and freedom from colour. Test 

 objects are used for this purpose, certain of these requiring a lens of 

 a definite quality to properly resolve their details. As examples of 

 test objects for medium powers I might mention those of the fine 

 hairs on the tongue of the blow-fly, hairs of Dermestes and Lagi/ri/s 

 for definition, section of pine-wood for achromatism, and spine of 

 Echinus for flatness of field, whilst for higher powers the secondary 

 markings on some of the finer diatoms, such as Surirella gemma, 

 Navicula rhomboides, some bacteria and their flagellar, and the well- 

 known Podura scale test. 



Objectives are termed low, medium, and high power. The low 

 power range from 4 in. to 1 in., medium from 1 in. to \ in., and 

 high power from \ to T V in. Measurements of their respective 

 foci are, however, better expressed in millimetres. It must be under- 

 stood that the terms 4 in., etc., do not represent the actual distance 

 between the front lens of the objective and the object, but bear an 

 approximate value according to the distance of the objective from 

 the eyepiece. Except for the study of bacteriology, when the use of 

 a -jV in. is imperative, it is seldom necessary that a higher power 

 than \ in. will be employed by the entomologist, whilst for general 

 work a 2-in. will be found the most useful all-round objective. For 

 the examination or photography of larger structures, such as wings, 

 or entire insects, or the larger eggs, a 3-in. or even 4-in. objective is 

 necessary, or, better still, a lens of the "Planar" type. These latter 



