408 SUMMARY OF CURRENT RESEARCHES RELATING TO 



position of the systems is only an expedient, yet if it can be sliowu 

 that it reaches the desired end more certainly, speedily, and accurately 

 than any other, the objection to it must fall to the ground. 



Third, that while it is conceded that really first-class metal-work 

 is exjjeusive, if it can be shown that it is necessary, the objections to 

 it must also fall. 



The term homogeneous-immersion, though honestly applied and 

 correct as to the idea, is only approximately true at present, as no 

 truly homogeneous-immersion fluid has as yet been discovered, so far 

 as the author can learn. That is, no fluid whose optical properties 

 are absolutely identical with those of the front lens of any objective. 

 The refractive power of crown glass has been closely approximated, 

 but minute differences of dispersive power remain ; and even if this 

 difiiculty could be overcome, the varying refractive and disjDersive 

 powers of various samples of crown-glass must always remain an 

 unknown quantity in our problem, to be provided for by some kind of 

 adjustment. 



This fact has been recognized by at least one maker, who advises 

 to correct for exti-emely thick or thin covers, by means of the draw- 

 tube, and furnishes two fluids, one for use with direct central light, 

 and the other with very oblique light. Of course it follows that for 

 perfect accuracy of correction by means of the immersion fluid, a 

 different fluid would be needed for each degree of obliquity of illu- 

 mination. That this would involve serious inconvenience hardly 

 needs demonstration ; more esi^ecially when we consider that it is 

 often desirable to examine an object undev gradually varying obliquity 

 of illumination, from direct central to the most oblique the lens can 

 utilize. 



Another point is the variation in the human eye ; which must be 

 compensated for in some way. 



" It appears then, that the homogeneous-immersion system does 

 not entirely obviate the necessity for adjustments of some kind, though 

 it greatly lessens their extent. That these small residual adjustments 

 can be made with more ease, rapidity, and accuracy by means of the 

 screw-collar moving the back system of the objective, than by means 

 of varying the distance between the objective and eye-piece by means 

 of the draw-tube, or by varying the refractive and dispersive powers 

 of the immersion medium by means of mixtures of various oils, &c., 

 in varying proportions will, I think, on consideration be generally 

 admitted. 



But this greater ease, rapidity, and accuracy of adjustment with 

 homogeneous-immersion (so called), is not the only argument in 

 favour of the retention of the adjustable mounting for objectives. 

 Most immersion fluids are apt to vary in their optical properties with 

 their age or the state of tlie weather. One of the best of them, the 

 solution of the sulpho-carbolate of zinc in glycerine, has its refractive 

 power increased in very dry and decreased in very wet weather. In 

 this case it is more convenient to turn the adjustment-collar slightly, 

 than to make a new solution for immersion. 



Again, it is often desirable to use an objective with a much lunger 



