MICROSCOPIC OBJECTS. 



3 



shape of the microscope is immaterial; 

 both Ross and Jackson models will give 

 good results if well made. A graduated 

 draw-tube should be obtained, which had 

 best be velvet-lined, to prevent flare. The 

 usual dead black, after a while, wears out 

 of the draw-tube, and renders the micro- 

 scope useless for photography ; thus a more 

 durable material, such as cloth or velvet, 

 should be used to prevent reflection from 

 the sides of the tube. 



Several of the continental models, such 

 as Hartnack's, would be useful for photo- 

 micrography, from their compact shape and 

 solidity; but their narrow body-tube, 

 which limits the field of view most serious- 

 ly, and cramped stage, render it advisable 

 to use only English instruments of the 

 latest pattern. If cost be no consideration, 

 then there is nothing to equal one of the 

 large, first-class microscopes of the best En- 

 glish makers, fitted with every possible con- 

 venience in stage and sub stage. It is true, 

 an expert manipulator will obtain excellent 

 results with the simplest arrangements ; but 

 it is no less true that it is the expert alone 

 who can really appreciate and turn to good 

 account the delicate mechanical contiiv 

 ances which the skill of the optician has 

 devised for his aid. Thus, a mechanical 

 stage is not absolutely necessary, but it is a 

 great help when working with high pow- 

 ers ; and with the very highest powers it is 

 hard to see how it can be dispensed with. 

 The same may be said of the sub-stPge; 

 but as this is in some respects more gener- 

 ally useful than the mechanical stage, it 

 should be applied to all microsoopes with 

 which an achromatic condenser or parabo- 

 loid is to be used. The objections to the 

 tube- fitting usually supplied with student's 

 microscopes are, the difficulty of properly 

 adjusting the sub-stage apparatus, and the 

 very thick upper stage that they necessi- 

 tate. For photographic work, the upper 

 stage should be as thin as possible, certainly 

 not more than J-inch thick, for frequently 

 very oblique light must be employed, and 

 this cannot be done with a thick stage, 

 which cuts ofif the rays. Using a thin stage 

 and bull's eye lens, it is astonishing how 



easily a difUcult diatom may be resolved, 

 which, with a thick stage, would require the 

 use of an expensive condenser. Most of 

 the English makers now fit their micro- 

 scopes with thin concentric stages, even 

 when the rack and pinion movement is 

 omitted. 



As to lenses, the student is advised 

 strongly to buy the very best, if possible. 

 Let him shun cheap French lenses, more 

 especially the separating lenses, styled 

 "French buttons," which are frequently 

 supplied with or £(> microscopes, and 

 which are only useless rubbish. The stand 

 and lenses should be purchased separately, 

 the latter to suit the requirements of a pho- 

 to-micrographer. If the very best lenses 

 are too expensive, then purchase some of 

 the cheap low angle lenses, now sold by 

 most good makers, for these, being well cor- 

 rected up to the angle ascribed to them, 

 are capable of performing a vast amount of 

 real work. The beginner will probably 

 find them much easier to handle than lenses 

 of wider aperture, owing to their greater 

 penetration and working distance ; but the 

 more experienced worker will require, es- 

 pecially for photographing very minute ob- 

 jects, lenses of the widest possible angle. 

 Lenses of wide angle admit more light, and 

 have far greater resolving power than lenses 

 of low angle; but they have less working 

 distance ; and less penetration. The fact 

 that they almost touch the object, in many 

 cases, when in focus, forms no objection to 

 their use for photography, but it is annoy- 

 ing to have a lens, say a -^-inch, that will 

 only show the surfaces of objects. This ob- 

 jection, however, can be easily disposed of, 

 by using a contracting diaphragm such 

 as the "Davis Aperture Shutter," made 

 and sold by Mr. Collins, whenever pen- 

 etration is desired. The use of this 

 shutter renders a lens of widest angle equal 

 to any low angle lens, as far as penetration 

 is concerned ; while even with this shutter, 

 the wide angle lens will give superior defini- 

 tion and admit more light than a low angle 

 lens of the same focus. 



The choice of lenses will depend, in a 

 great measure, on the photographic work to 



