220 Ohjed-glasses and their Definition. 



the object-glass. By means of the f rds object-glass and draw-tube 

 a power from 5 to 150 may be obtained. From the crossing of the 

 rays and the formation of an intermediate image, colour correction 

 was not impaired throughout, though for the best definition Mr. 

 Lister recommended an increase of distance between the two lenses 

 in proportion as the draw-tube was extended, and a means was pro- 

 vided for effecting this. "With a 1^-inch object-glass, as the tube is 

 closed the anterior focus becomes lengthened till its distance is 

 infinite, giving all the grades between a microscope and telescope, 

 and distant objects may be drawn with the camera lucida. A form 

 of dissecting microscope was manufactured by Smith and Beck on a 

 modification of this plan. The draw-tube was moved by a rack and 

 piaion. On viewing some injections with this instrument I was 

 struck by its extreme handiness ; for, by keeping one hand on the 

 lengthening and the other on the focussing pinion, all the ranges 

 from a low to a high power could be instantly made with one 

 object-glass, and the extent of field gave a great advantage over the 

 plan of changing the eye-pieces, and the images were sufficiently 

 " aristokratic " to make it well worth while to attempt to improve 

 the arrangement, and to ascertain how fer one object-glass of large 

 aperture may be made to do the work of several powers, for a cheap 

 form of insti-ument. 



Since the revival of this question I have made a number of 

 trials of the effects of intermediate lenses, either under or over- 

 corrected, in neutralizing any errors in the object-glass. Chromatic 

 correction cannot be obtained this way for the reasons stated, and 

 out of the numerous combinations that I have tried none have any 

 material influence in improving the oblique pencils ; a ith with 

 outward coma had outward coma still with all. If the object-glass 

 is carefully corrected by the adjusting collar on a covered object, 

 and any intermediate lens or lenses are then introduced, that cor- 

 rection will no longer serve, but has to be altered, as the searcher 

 (so to term it) itself induces an error of correction in the ohject- 

 glass, by shortening the posterior or conjugate focus and extending 

 the anterior one, and the aperture or angle of rays for which it was 

 specially corrected entering at different incidences to those previous 

 to the introduction of the intermediate lenses, positive aberration * 

 is caused ; this requires for every difference ia their position another 

 alteration of the adjusting collar. Sometimes the range of this is 

 not sufficient to produce the required correction — the " searcher " 

 may find a position with an object-glass improperly corrected or 

 adjusted, or ^nth. no means of adjustment at all, ia which the defi- 

 nition may be improved ; but in cases where a good object-glass 



rection 

 error 



* Dr. Pigott defines positive and negative aberration as over and under cor- 

 tion. Fact is the reverse. This naay, perhaps, be attributed to a printer's 



•»r- 



