THE MODERN USE OF COLLAE CORRECTION 359 



intervals between the points * covered ' and ' uncovered ' might be 

 recorded. 



The second, a more serious defect, was the movement of the 

 front lens while the back remained rigid with the body of the 

 microscope. The detriment of this arrangement was that in cor- 

 recting a wide-angled, close-working objective there was a danger of 

 forcing the front lens through the cover-glass by means of the collar 

 correction. 



Now the arrangement as shown in fig. 313 enables the front 

 lens to maintain a fixed position, while the correctional collar acts 

 on the posterior combinations only. This device was introduced 

 by Mr. F. H. Wenham in 1855. 



On the Continent it has beeyi the practice to graduate the cor- 

 rectional collar in terms of the thickness of the cover-glass in deci- 

 mals of a millimetre. Thus if a cover-glass be 0*18 mm. thick, the 

 correctional collar should be set to the division marked 0'18. 



In England, on the contrary, the divisions are entirely empiri- 

 cal, so that the operator has to discover for himself the proper 

 adjustment. It is not to be supposed, however, that the English 

 method is unscientific, for when an operator becomes expert he 

 would never for an instant think of adjusting by any other indi- 

 cation than that afforded by his own eye and experience. This is a 

 very important point, because the interpretation of structure to a 

 great extent depends on accurate adjustment of the objective, and 

 it would be folly to suppose that an eminent observer would sur- 

 render his judgment to the predetermination of theory embodied in 

 what must be the imperfections in even the most conscientious and 

 thorough work which gives a practical form to such theory. In 

 fact, it is the test of accurate manipulation that, however the collar 

 correction be disturbed, the microscopist will, in getting a critical 

 image of the same object, always, by the quality of the image he 

 obtains, bring the correction to within the merest fraction of the 

 same position, although the correction collar and its divisions are 

 never looked at until the desired image is obtained. 



The fact that the over-correction caused by the cover-glass was 

 discovered in England, and that means were at once found for its 

 correction, while no similar steps were taken on the Continent, is a 

 sufficient evidence of the advanced position of this country in practi- 

 cal optics at that time. 



This subject of under- and over-correction is one of large impor- 

 tance, and it may be well at this point to enable the tyro to clearly 

 understand, by evidence, its nature, although what it is has been 

 fully shown in Chapter I. Take a single lens the field-lens of a 

 Huyghenian eye -piece will serve admirably and hold it a couple 

 of yards from a lamp flame; the rays passing through the peri- 

 pheral portion of the lens will be found by experiment with a card 

 to be brought to a focus at a point on the axis nearer the lens than 

 those passing through the centre. This is under -correction, vide fig. 23, 

 p. 20. The same, experiment should be repeated with the plane 

 .side and the convex side of the lens alternately turned to the flame. 

 In the former case, when the image of the flame is at its best focus, 



