HI ROM on. 



MH ROMOPK 



i under eorreeUd peoeil from the front glaei, and to oounUr- 

 rrror by over-rorrectioo in the middle one. 



,-ht error* ill ooluur may in the same manner be destroyed by 



oppouu one* ; and on the principle* described we not only acquire 



BJM oorreotion for the central ray. but, by the opposite effect* at the 



i on the transverse pencil, all coma can be destroyed, and the 



whole field rendered beautifully flat and distinct," 



Mr. Lister'* paper enter* into further particulars, which are not 

 essential to the comprehension of the *ubjoct. It i* sufficient to any 

 that hi* investigation* and result* proved to be of the highest value to 

 the practical optician, and the progre** of improvement was in con- 

 Mqueooe extremely rapid. The new principle* were applied and 

 exhibited by Mr. Hugh Powell and Mr. Andrew Ron with a degree of 

 iooce** which had never been anticipated ; so perfect indeed were the 

 eorreotiuox given to the achromatic object-glad *o completely were 

 the errors of sphericity and dispersion balanced or destroyed - that the 

 circumstance of covering the object with a plate of the thinnest glass 

 or talc disturbed the corrections, if they had been adapted to an un- 

 covered object, and rendered an object-glass which was perfect under 

 one condition sensibly defective under the other. 



Thi defect, if that should be called a defect which arose out of 

 improvement, wa* first discovered by Mr. Rosi, who immediately 

 suggested the means of correcting it, and presented to the Society of 

 Art*, in 1887, a paper on the subject, which was published in the 51st 

 volume of their Transactions, and which, as it is, like Mr. Lister's, 

 essential to a full understanding of the ultimate refinements of the 

 instrument, we shall extract nearly in full. 



"In the course of a practical investigation (say a Mr. Ross) with the 

 view of constructing a combination of lenses for the object-glass of a 

 compound microscope, which should be free from the effects of aber- 

 ration, both for central and oblique pencils of great angle, I combined 

 the condition of the greatest possible distance between the object and 

 object-glass ; for in object-glasses of short focal length their closeness 

 to the object has been an obstacle in many cases to the use of high 

 magnifying power*, and isa constant lonrce of inconvenience. 



" In the improved combination, the diameter is only sufficient to 

 admit the proper pencil ; the convex lenses are wrought to an edge, 

 and the concave have only sufficient thickness to support their figure ; 

 consequently, the combination is the thinnest possible, and it follows 

 that there will be the greatest distance between the object and the 

 object-glass. The focal length is J of an inch, having an angular 

 aperture of 60", with a distance of ^ of an inch, and a magnifying 

 power of 970 times linear with perfect definition on the most ditlicnlt 

 Podura scale*. I have made object-glasses . ; of an inch focal length ; 

 but a* the angular aperture cannot be advantageously increased, if the 

 greatest distance between the object and object-glass is preserved, their 

 use will be very limited. 



" The quality of the definition produced by an achromatic compound 

 microscope will depend upon the accuracy with which the aberrations, 

 both chromatic and spherical, are balanced, together with the general 

 perfection of the workmanship. Now, in Wollaston's doublets, and 

 Holland's triplets, there are no means of producing a balance of the 

 aberrations, as they are composed of convex lenses only ; therefore the 

 best that can be done is to make the aberrations a minimum : the 

 remaining positive aberration in these forms produces its peculiar effect 

 u|m object* (particularly the detail of the thin transparent class), 

 which may lead to misapprehension of their true structure ; but with 

 the achromatic object-gla**, where the aberrations are correctly 

 balanced, the meet minute parts of an object are accurately displayed, 

 o that a latiafactory judgment of their character may be formed. 



V 



" It will be seen by Jig. 1 5, that when a certain angular pencil A o A' 

 proceeds from the object o, and is incident on the plane side of the 

 flnt Urn, If the combination i* removed from the object, as in 

 the extreme ray* of the pencil impinge on the more marginal part* of 

 the glass, and as the refraction* are greater here, the aberration* will 

 b* greater also. Now, if two compound object gbuwos have their 

 aberration* balanced, one being situated as in >;. 15, aixl 

 in .Hj. 11. and the came disturbing power applied to both, that in which 

 the angle* of incidence and the aberrations are small will not be so 

 much disturbed as where the angle* are great, and where consequently 

 the aberration* increase rapidly. 



".When an object-glass ha* it* aberration* balanced for viewing an 

 opaque object, and it is required to examine that object by trans- 

 mi tU>.l light, the correction will remain; but if it is necessary to 

 immerse the object in a fluid, or to cover it with glass or talc, an 

 aberration will arise from these circumstance*, which will disturb the 

 previous correction, and consequently deteriorate the definition ; and 

 this effect will be more obvious with the increase of the 

 between the object and the object-glass. 



Fig. 17. 



" The aberration produced with diverging rays by a piece of flat and 

 parallel glass, such as would be used for covering an object, is repre- 

 sented at jig. 17, where o o oo is the refracting medium, or piece of 

 glass covering the object o ; o P, the axis of the pencil, perpendicular 

 to the flat surfaces; OT, a ray near the axis; and or', the rv 

 ray of the pencil incident on the under surface of the glass : then T it, 

 T' K', will bu the directions of the rays in the medium, and it E, it' >:', 

 those of the emergent rays. Now if the course of these rays is con- 

 tinued, as by the dotted lines, they will be found to intersect the axis 

 at different distances, X and Y, from the surface of the glass ; and the 

 distance X T is the aberration produced by the medium which, aa 

 before stated, interfere* with the previously balanced aberrations of 

 the several lenses composing the object-glass. There are many canon 

 of this, but the one here selected serves best to illustrate the principle. 

 I need not encumber the description with the theoretical determination 

 of this quantity, as it varies with exceedingly minute circumstances 

 which we cannot accurately control ; such as the distance of the object 

 from the under side of the glass, and the slightest difference in the 

 thickness of the glass itself ; and if these data could be readily obtained, 

 the knowledge would be of no utility in making the correction, that 

 being wholly of a practical nature. 



" If an object-glass is constructed as represented in fy. 16, where the 

 posterior combination P and the middle V have together an excess of 

 negative aberration, and if this be corrected by the anterior combination 

 A, having an excess of positive aberration, then this latter oombi- 

 can be made to act more or lees powerfully upon P and M, by making 

 it approach to or recede from them ; for when the three are in close 

 contact, the distance of the object from the object-glass is greatest ; 

 and consequently the rays from the object are diverging from a point 

 at a greater distance than when the combinations are separated ; and 

 as a lens bends the rays more, or acts with greater effect, the more 

 distant the object is from which the rays diverge, the effect 

 anterior MuUatUm A upon the other two, p and M, will vary with its 

 distance from thence. When, therefore, the correction of the whole is 

 effected for an opaque object with a certain distance between the ante- 

 rior and middle <-oinl>inRtion, if they are then put in contact, the 

 di-t mee between the object and object-glass will be increased : conse- 

 quently the anterior combination will act more powerfully, and the 

 whole will have an excess of positive aberration. Now the effect of 

 the aberration produced by a piece of flat and parallel glass being of 

 the negative character, it is obvious that the above considerations 

 suggest the mean* of correction by moving the lenses nearer together, 

 till the positive aberration thereby produced balances the negative 

 aberration caused by the medium. 



" The preceding refers only to the spherical aberration, but the effect 

 of the chromatic is also seen when an object is covered with a piece of 

 glas* ; for, hi the course of my experiments, I observed that it pro- 

 duced a chromatic thickening of the outlines of the Podura and other 

 delicate scales ; and if diverging rays near the axis and at the margin 

 ore projected through a piece of flat parallel glass, with the various 

 indices of refraction for tin- ditti n nt colours, it will be seen that each 

 ray will emerge se]iar.it<-d into a beam consisting of the component 

 colours of the ray, and that each beam is widely different in form 

 difference, being magnified by the power of the microscope, readily 



' 1 for the chromatic thickening of the outline ju.-i 

 Therefore, to obtain the finest definition of extremely dclica' 

 minute objects, they should lie viewed without a < : it be 



desirable to immerse them in a fluid, they should be covered with the 



