COERESPONDENCE. 1 09 



index, before the commencement of a measurement, exactly opposite a 

 division of the scale ; in this way there are no fractions to be noted 

 down at the starting point, but only at the end of the operation. 



A turn of the micrometric screw of my microscope being equal to 

 half a millimetre, each division of the scale, in consequence of the 

 whole being divided into 100 parts, is equivalent to tj-^^^ of a millimetre. 

 In order to measure the thickness of a round thin glass cell cover for 

 example, I begin by drawing with a diamond a very small stroke on 

 the edge of one of its surfaces. I make a similar mark on the opposite 

 surface, in such a way that the two marks may form a cross ; this is 

 in order not to confound them. If the glass is square, the cross will fit 

 much more easily in one of the angles. One of the marks (the upper 

 one) is then brought into focus, and the division of the scale opposite 

 the index noted down. The body of the microscope is then moved down 

 by turning the head of the screw from left to right, until the second 

 mark comes into focus, while the first disappears. I now find out the 

 thickness of the glass by ascertaining how many divisions of the scale 

 have been passed over, counting from the starting point already noted. 

 As regards microscopic objects, however thin they may be, provided 

 they have two faces distinct enough, I bring one face into focus as 

 starting point, then examine the scale of the screw, and proceed as 

 before described. I measure in this way the thickness of diafomacece, 

 those interesting little beings with silicious envelopes, whose stricp, to 

 use the old term, are not in the same focus. When I wish to show a 

 preparation to anyone not much accustomed to use a microscoj)e, this 

 process enables me to regulate the focussing for other objects also, 

 after I have once determined the difference between our sights, by 

 means of an examination, made by us both, of the first preparation. I 

 can also determine with much greater facility whether the focus of 

 both eyes differs in either of us, and in what proportion. 



I come naturally to spectacles next. When the sight is not in its 

 normal state, biconvex and biconcave lenses, with both faces similarly 

 curved, are usually employed to remedy imperfections arising from 

 defective refrangibility of the eye, or from disturbance in its accom- 

 modating power. Lenses of those kinds are the most powerful, their 

 construction is the simplest, and the focal distance is most easily cal- 

 culated, as it is equal to the radius. Each of the two surfaces of those 

 lenses is a segment of a sphere of known diameter. The shorter it is, 

 the greater is the convexity or concavity. 



Medium sight is estimated at 10 English inches, or 25 centimetres. 

 By experience, I find my own to be 30 inches, or 75 centimetres. Those 

 both being known serve as the basis of the following calculation. I 

 multiply 10 inches (the number in normal or medium sight) by 30 

 (that is my own), and find as result the number 300, which, when 

 divided by the difference, that is by 20, gives 15 inches as quotient. 

 This quotient indicates both the focal distance of the lens suitable for 

 my sight, and the radius of the sphere, which is the same thing. We 

 speak of inches, because the lenses of spectacles are still marked 

 according to the old measure. This method of measuring the focal 



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