SciEyTiFic Lectures. 39 



with effect, in investigating, single lenses of the large power, requires 

 consummate skill. If the power is great, the lens must be exceed- 

 ingly small. The image, except exactly in the center, is greatly dis- 

 torted. The worJdng distance is very small, both for the object and 

 the eye. All these objections are obviated by the use of the com- 

 jpound micToscope. 



In the compound microscope an image is fonned of an object by 

 one convex lens, and this image is observed by a second. The first 

 image is much greater than the object, and this being a second time 

 magnified, the resultant effect is expressed by the product of the num- 

 bers representing the sepai'ate effects. If the object glass magnifies 

 twenty times, and the eye glass five times, the total enlargement will 

 be 100. The image is inverted relativeh' to the object, but this is 

 unimportant, as generally the object itself may be placed in any posi- 

 tion. The form of the compound microscope facilitates greatly the 

 correction of the aberrations of sphericity and color, and that of the 

 curvature of the field and the distortion of the image. Though the 

 two lenses only are required, in order that we may state the principle 

 of the instrument, a number are necessary for the purpose of affecting 

 the corrections just mentioned. In the first place, the aberrations are 

 much reduced by employing, instead of a single lens for the object 

 glass, a combination of three. Aberrations increase rapidly with 

 curvature. Three lenses of low curvature will have much~ less 

 than a single one, equivalent in magnifying power to the three 

 jointly. The object viewed is placed within the principal focus of 

 the fii*st of the three forminor the combination. The nescative imasre 

 thus resulting, is the object of the second lens, and falls within its 

 principal focus also, A second negative image thus produced falls 

 without the principal focus of the third lens, and this lens produces 

 finally a positive image to be observed by the eye-glass. Another 

 very important end is attained by making the object-glass compound. 

 "When a single lens has been corrected for the aberrations of spheri- 

 city, the coiTection will be good, only for certain distances of the object 

 from the lens. There are, in fact, only two points for which such a lens 

 will be aplanatic (without abeiTation). If the object be placed any- 

 where between these two points, the aberration will be over-corrected, 

 i. e.^ the foci of marginal rays will be too long, and those of axial 

 rays too short. But if the object be more distant from the lens than 

 the more remote of these two points, or less distant than the least 

 remote, the aberration will be under-corrected. In the compound 



