COMPOUND MICEOSGOPE 39 



he the most powerful of all simple microscopes, and the space 

 available for scalpels, needles, &c. will be much greater than 

 with a doublet alone. The further the concave lens is removed from 

 the latter, the greater will be the amplification. 1 Even in this, 

 however, Chevalier had been anticipated by Professor Joblot in 

 1718. 



This combination, applied to lenses for examining the eye and 

 skin, allows the use of doublets which leave a considerable distance 

 above the object, and it is this idea which has governed the con- 

 struction of the Briicke lens. 



' The lens has a very long focus, and the construction is that of 

 the Galileo telescope as applied to opera-glasses, but the amplifica- 

 tion of the objective is much greater than that usually obtained in 

 opera-glasses. The focus is about 6 cm., and the power three to 

 eight times. The latter power is obtained by lengthening the tube, 

 by which means the distance between the two lenses is much 

 enlarged, and the amplification increased without inconveniently 

 modifying the focus.' 



This lens may be used in place of the body of a compound 

 microscope, when it is desired to dissect or to find small objects, or 

 it can be adapted to a simple microscope or lens-holder, with from 

 3 to 8 cm. between the object and objective. But the Briicke lens, 

 like the Galilean opera glass, has a very small field. 



Compound microscope. The compound microscope, in its most 

 simple form, consists of only two lenses, the object-glass and the 

 eye-ylass, and is a Keplerian telescope adapted for viewing verv near 

 objects. The former receives the light-rays direct from the object 

 brought into near proximity to it, and forms an enlarged but inverted 

 and reversed image at ;i greater distance on the other side ; whilst the 

 latter receives the rays which are diverging from this image, as if 

 they proceeded from an object actually occupying its position and 

 enlarged to its dimensions, and brings these to the eye, so altering 

 their course as to make that image appear far larger to the eye, pre- 

 cisely as in the case of the simple microscope. It is obvious that, 

 in the use of the very same lenses, a considerable variety of magnify- 

 ing power may be obtained by merely altering their position in regard 

 to each other and to the object. For if the eye-glass be carried farther 

 from the object-glass, whilst the object is approximated nearer to the 

 latter, the image will be formed at a greater distance from the object- 

 glass, and the dimensions of the magnified image will consequently 

 be augmented ; whilst, 011 the other hand, if the eye-glass be brought 

 nearer to the object-glass, and the object removed further from it, 

 the distance of the image from the object-glass will be less than it 

 was before, and the dimensions of the magnified image will be 

 correspondingly diminished. The amplification may also be varied 

 by altering the magnifying power of the eye-pieces. In practice, 

 variations in power must be obtained by altering either the objective 

 or the eye-piece, or both, and the use of the draw-tube for this 

 purpose must be altogether abandoned, because objectives are 



1 Robin, C., Traite clu Microscope et des Injections, 2nd ed. 8vo. pp. 33, 34. 

 Paris, 1887. 



