658 



MICROSCOPE. 



portance and requires to be reckoned in the focal 

 length. The foci of ordinary thin lenses are rec- 

 koned thus ; that of a plano-convex at the distance 

 of a diameter, and that of an equal and double con- 

 vex at the distance of a radius, of the sphere of 

 which their respective surfaces are portions. The 

 focus of a sphere is at half the radius, but to this 

 must be added a whole radius, when we wish to 

 ascertain a ratio between its amplifying power and 

 that of the human eye. Hence it will be eeen 

 that a double and equally convex lens has a 

 greater magnifying power than the sphere of 

 which its surfaces are portions. As, however, we 

 can produce spheres of much smaller radius than 

 any double convex lens, it is a great advantage to 

 have the means of correcting their performance as 

 magnifying powers. Hemispheres have two foci : 

 if the convex side be towards the object, the focus 

 is at a radius and a third from the refracting sur- 

 face; but if the plane side be exposed to the ob- 

 ject, the focus is then at the distance of two radii ; 

 and hence it is discovered that in either case the 

 hemisphere has more than half the magnifying 

 power of the sphere, and that the exposure of the 

 plane surface gives the least power of the two. 

 The same rule holds proportionally with regard to 

 the plane and convex sides of any plano-convex 

 lens. It may be asked, if the exposure of the 

 convex surface gives a greater power, why does the 

 optician, always place the plane side towards the 

 object? Simply, to correct in a good degree the 

 spherical aberration, and to effect the transmission 

 of a larger pencil of light, two important points, 

 which more than compensate for the reduction of 

 power. From what we have observed concerning 

 the foci of common lenses, it will be obvious that 

 there is a peculiarity about those of the doublet 

 and triplet magnifiers. The focus of a doublet 

 measures from between the two lenses, conse- 

 quently if the focal length be one-twentieth of an 

 inch, the anterior lens must be brought considera- 

 bly within that distance of the object ; and here 

 we discover an objection to the deep triplet, which, 

 from being composed of three lenses, gives a very 

 short focal distance between the anterior surface 

 and the object, and comes down inconveniently 

 close upon the slide. The doublet appears, for 

 the reason above stated, to be better adapted for 

 extreme depth of power than the triplet, which in 

 addition to its inconvenient focus loses much of the 

 light by reflection from the number of surfaces. 

 Mr Gary informs us, " that he has always found a 

 good doublet fully equal in defining power to any 

 triplet." 



It will not be out of place to remark briefly on 

 the present rage for deep magnifying powers. The 

 minute atoms which are now subjected to micro- 

 scopic examination, and the vast real improvements 

 and advances in the elements of the instrument, 

 seem to have suggested an idea that the amplify- 

 ing power of the microscope may be carried to 

 almost any extent with proportionately increased 

 advantages to the observer. The fact is, however, 

 that with all the benefits resulting from improved 

 construction, and the adoption of media of higher 

 refractive powers, we have still to guard against 

 the error of limited aperture and loss of light. 

 There appears to be little advantage, so long as 

 glass is the refractive medium employed, in using 

 single lenses of less than one-twentieth inch focus, 

 or doublets of deeper power than one-thirtieth 

 inch focus. Any thing which they fail to discover 



will, we believe, be very uncertainly shown by 

 deeper amplifiers of the same medium. Indeed, as 

 an evidence that microscopic discovery does not 

 depend altogether on depth of power, we may 

 mention that we have a double convex lens of one- 

 tenth inch focus, and one-twentieth inch aperture, 

 that shows the markings on the larger scales of the 

 podura with great distinctness, and this object has 

 been, and still is, considered a very severe test of 

 defining and penetrating power. When jewels are 

 employed as the refracting media, of course the 

 amplifying power may be advantageously increased 

 in proportion to their refractive properties. It is 

 however a question whether we should not take 

 the great advantages of large aperture and addi- 

 tional light which they offer, rather than push the 

 amplifying power to its extreme point. 



Some difference of opinion still exists respecting 

 the proper mode of expressing the magnifying 

 power. The micrographers of the present day for 

 the most part seem to incline towards the linear 

 measure ; but there are some who prefer expressing 

 the power by the magnified surface, and other few 

 who contend for the whole cube. Now it is obvi- 

 ous, that for a mere comparison between different 

 magnifiers it is quite sufficient to state the dia- 

 metric enlargement, as that gives the simple ratio 

 of their powers; but we conceive the linear mea- 

 sure isnotat all satisfactory when we would compare 

 surfaces, unless we mentally, or otherwise, calcu- 

 late the square, in which case we might as well 

 express the magnifying power at once by the su- 

 perficial enlargement ; and again, the square will 

 fail to give us just ideas of the relative bulk of any 

 two bodies, as this can only be ascertained from 

 the cubic content of each. We are aware that an 

 attempt on our part to revive the practice of com- 

 puting by the cube would raise a laugh against us ; 

 yet, whilst we admit that to say of a one-thirtieth 

 inch doublet, it magnifies twenty-seven millions 

 times, would savour of charlatanism, we must still 

 remind observers that such a doublet really shows 

 a minute object so highly magnified, that twenty - 

 seven millions of such objects condensed into one, 

 would expose a visible surface to the naked eye of 

 no greater extent than that presented by the mag- 

 nified object. With respect to the magnified sur- 

 face, we are of the number of those who contend 

 for it as the true measure of the magnifying power : 

 the eye computes by surfaces not lines, and it is 

 most absurd to say that we exaggerate the perfor- 

 mance of microscopes by using any other than the 

 linear measure. The preceding table, however, 

 exhibits the powers on the line, surface and cube ; 

 and our readers can adopt the measure most satis- 

 factory to themselves. 



MICBOMETER. Our former article under ibis 

 head refers only to the micrometrical adjustments 

 of telescopes; but it will be obvious that a modi- 

 fication of the same principle of construction applies 

 equally well to the microscope. The name of the 

 instrument sufficiently explains its use, viz., to take 

 exceedingly small measurements, with great deli- 

 cacy and exactness. In the case of the microscope, 

 the micrometer is designed to measure the actual 

 dimensions of those minute objects that are either 

 barely perceptible by unaided vision, or which from 

 their smallness, are altogether concealed from the 

 unassisted eye. It would unnecessarily extend the 

 present article, to enter into detailed explanation 

 of the various contrivances that are in use for this 

 purpose ; we may simply state, that they consist 



