656 



MICROSCOPE. 



A 11 C is a hemispherical plano-convex lens. 

 From an inspection of the diagram, it will be evi- 

 dent that a pencil of rays proceeding from any 

 point in an object at O, will be refracted at the 

 surface B E C, reflected from the plane surface 

 A C in correspondent angles, and, after suffering an- 

 other refraction at the surface A B, proceed paral- 

 lel. Consequently an eye at D will see the object 

 magnified twice as much as it would have been by 

 the lens, ABC used in the ordinary manner ; | 

 since the object is in fact magnified by the two 

 equal plano-convex lenses B E C, A D B, whose 

 focal distance, when acting together, is the radius of 

 the sphere of which they, in common with ABC, 

 are parts. 



Fluid Lenses. By a fluid lens is to be under- 

 stood a small drop of water, or other fluid, sus- 

 pended in a small aperture in a plate of brass or 

 silver. It will be obvious that a lens of this na- 

 ture takes an irregular form, the upper surface 

 being somewhat flatted, and the lower one having 

 an elongated sphericity. The construction of this 

 description of lenses is far from being perfect; and 

 we make mention of them in this place only for the 

 purpose of alluding to the scientific efforts of Sir 

 D. Brewster in this department, which may be ex- 

 pected to issue in the formation of fluid lenses pos- 

 sessing the same advantages with those constructed 

 from gems, and having even less eccentricity of 

 action than they have. 



COMPOUND MAGNIFIERS. It remains only, in this 

 department of our subject, that we give a popular 

 description of the compound magnifiers at present 

 in use, named Doublets and Triplets, and we shall 

 confine ourselves to a single example of each. 



Wollaston's Doublet The objects contemplated 

 in this contrivance are deep power, unusual extent 

 of field, and a diminution of chromatic and spheri- 

 cal aberration. This doublet, as first constructed 

 by Dr Wollaston, consists of two piano convex 

 lenses, the focus of the one being to that of the 

 other, as 3 to 1. The lens of shortest focal length 

 is fixed with its plane surface towards the object, 

 and the other lens is placed over it at a certain dis- 

 tance, with the convex side towards the eye. Mr 

 Pritchard has corrected some apparent errors re- 

 garding the nature and construction of this magni- 

 fier, and Sir D. Brewster has experimented upon 

 it with his usual talent. The action of the Wollas- 

 ton Doublet when brought to bear, by a delicate ad- 

 justment, on minute transparent objects, is most 

 admirable, and its defining power is worthy of all 

 praise. 



Pritchard's Triplet. This is a construction 

 modelled on the plan of the Doublet, and depen- 

 dent on the same principles. As the name implies, 

 it consists of three lenses, the third (the one 

 nearest to the eye) having a greater focal length 

 than the other two. When properly centred and 

 adjusted, this combination acts very powerfully in 

 defining and penetrating the details of that difficult 

 class of objects named the Tests. It will of 

 course be understood that the gems frequently 

 enter into the composition of doublets and triplets, 

 when, if the selection be judicious, their perfor- 

 mance is exceedingly powerful and accurate. We 

 merely add, that the magnifiers just described can 

 be used to advantage only with microscopes of very 

 perfect adjustment; nothing satisfactory can be 

 shown by them unless the focus be most accurately 

 obtained. 



THE MANAGEMENT OF LIGHT AND ILLUMIN- 



ATION. The improved microscopic, elenii'i. 

 which we have briefly alluded, depend for their pro- 

 per and satisfactory performance on a judicious con- 

 densation and adaptation of light. We shall not dis- 

 cuss the subject of monochromatic illumination, as it 

 is a refinement in science of small regard in a popu- 

 lar essay, and is as well little more at present than 

 a suggestion by Sir D. Brewster which may issue 

 in some useful practical results. Neither shall wu 

 stay to consider the effects produced by polari/ing 

 the light admitted to the microscope, as this again 

 is one of the minutiae in microscopic observation 

 that scarcely comports with the popular use of the 

 instrument. It is sufficient for our purpose to re- 

 mark upon the reflectors and condensing lenses 

 ordinarily employed in the.supply and management 

 of the light. Compound microscopes of the com- 

 mon construction are frequently provided with no 

 other means for adapting the light than a concave 

 speculum ; but as these instruments are usually 

 contrived so as to combine the principles of both a 

 simple and compound microscope, this reflector is 

 insufficient to produce the desired effect; for when 

 used with a single lens, it converges the rays of 

 light too much to admit of their filling the field of 

 view, and it produces also an unpleasant glare in 

 the centre, that disturbs and confuses the image. 

 When the compound body is screwed into the object 

 lens, the concave reflector answers well, as its con- 

 vergence of the rays causes a more intense light to 

 pass through the diaphragm, or small aperture, of the 

 tube, which consequently can better afford the 

 attenuation which takes place in the expanded 

 compound field, than the light which is thrown up 

 by a plane reflector. These microscopes ought, 

 therefore, to be furnished with two reflectors, one 

 plane and the other concave ; otherwise their adap- 

 tation for simple and compound powers is imper- 

 fect. A plane reflector alone, will answer the pur- 

 pose, if a condensing lens be fitted to the pillar of 

 the instrument so that it may intercept the rays 

 when required, and be made to slide up or down to 

 converge them more or less according to circum- 

 stances. The condensing lens will serve another 

 important purpose; for by a simple contrivance it 

 can be made to concentrate a strong light upon 

 opaque objects, without which they could not be 

 distinctly seen. It is to be remarked that good 

 day-light answers best for opaque objects under a 

 moderate power of the simple microscope; but 

 when it is wished to examine them with deep mag- 

 nifiers, an artificial light strongly condensed is re- 

 quired. For transparent objects, natural light is, 

 speaking generally, not well adapted ; they come 

 up with a better definition and altogether more sa- 

 tisfactorily when the concentrated light of a lamp 

 or candle is transmitted through them. We have 

 ourselves seldom used the compound microscope 

 with natural light for any object or under any 

 power; and we believe few observations can be 

 made to advantage with it ; as the strongest day- 

 light will scarcely afford sufficient light for the 

 field even when the object is transparent, and for 

 opaque objects it is, under the -lowest compound 

 power, by far too weak. These general observa- 

 tions apply to the ordinary instruments in use 

 amongst the many, and will not hold altogether 

 with reference to those expensive and elaborate 

 constructions and adjustments which are furnished 

 by the eminent opticians of the present day, to the 

 few who can afford to purchase them. 



The advantage of using good day -light, so far as 



