of the Microscope. 427 



cross the axis after refraction at the field-glass, and z that where 

 it actually crosses after emerging from the eye-glass, the angle 

 of flexure, at each lens, is double of the original inclination of 

 the pencil to the axis. 



This simple system is however not applicable, as it is im- 

 possible to satisfy the condition necessary for perfect distinctness, 

 much less that for destroying, as far as possible, the convexity 

 of the field. These may however be very readily satisfied by 

 employing two lenses of equal power, in each place, instead of 

 one. The most proper forms of the lenses are those shown in 

 Fig. 2, the field-glasses and the second eye-glass being of the 

 meniscus form, and the first eye-glass equi-convex. I have found 

 no sensible error arise from the substitution of plano-convex lenses 

 for the meniscus glasses, which are diflicult and expensive to 

 form. Theory indicated a further flattening of the field, to be 

 made by separating the eye-glasses a little, which requires the 

 distance of the first eye-glass from the field-glasses, to be dimi- 

 nished by about half as much*; I cannot say however that 

 I perceive any improvement arising from this alteration in practice, 

 and as the field is quite flat enough with the eye-glasses in 

 contact, and any further diminution of the apparent convexity, 

 can be gained only by a sacrifice of distinctness, I cannot on 

 the whole recommend it. I have not however yet had the 

 instrument in a sufiiciently perfect state of adjustment, in other 

 respects, to be able to give a decided opinion on this point. 

 This system, as it will easily be seen, gives a magnifying power 

 of 3 to the eye-piece, so as to multiply, by that number, the 

 power of the object-glass. It would be easy, if necessary, to 

 produce a higher magnifying power, by employing lenses of 



• The first eye-glass should in this case, be a little less curved on the lower surface 

 and a little more on the upper, but it is hardly worth while to alter the form in practice. 



