'-".IS SUMMARY OF CURRENT RESEARCHES RELATING TO 



construction of objectives, and eye-pieces, and the possible and real 

 relations of each to the other. There is nothing to bo found indeed in 

 our language (except in the invaluable translations published in the 

 successive Journals of the Royal Microscopical Society) which discusses 

 the phenomena of diffraction, of polarization, of the principles of the 

 true interpretation of microscopical images, and the theory of work with 

 the Microscope. English workers with high powers have discovered 

 painfully where their lenses during many years were at fault ; they 

 could show our opticians what they wanted ; but it has been only as the 

 result of the laborious mastery of the theory of lens-construction by 

 German investigators, with Abbe at their head, that the English worker 

 has been able to get his wants, in object-glasses and eye- pieces, 

 supplied. 



" But like all advances in insight and analytical power, these very 

 improvements, so welcome and so helpful to searchers in many important 

 branches of science, only open up the horizon of the unknown more fully ; 

 and the very knowledge we get, through the inestimable improvements, 

 only reveals new difficulties ; and again creates optical wants. It is 

 then, with pleasure indeed that we hail this excellent translation of 

 Nageli's work on the theory and practice of the Microscope." 



Fasoldt's Test-plates. — A good deal of amusement has been felt in the 

 Old World at the vagaries of part of the New over these plates. As Old 

 World microscopists are aware, it is one of the plainest and best esta- 

 blished scientific truths that there is a limit to the number of lines to the 

 inch that can be made visible to the human eye with our existing optical 

 appliances, and to believe that more have been seen relegates the believer 

 to the ranks of those who believe in perpetual motion, the creation of 

 force, squaring the circle, and other self-demonstrated fallacies. 



Our American brethren are not one whit behind us in their apprecia- 

 tion of scientific principles, and it was therefore puzzling to read from 

 time to time positive statements that many people had seen 200,000 lines 

 to the inch — the limit, even with the maximum aperture of 1 ■ 52, being 

 158,845. We put out of account the statements of the ruler of the 

 lines, as he may be forgiven a not unnatural tendency to see lines that 

 he feels certain his acknowledged mechanical skill has really put on the 

 slide. 



We gather that the explanation of these discrepancies is that the 

 persons who are " ready to make affidavits " that they saw the lines are 

 people who have had no practice in such observations, and it is well 

 known how much the power of recognizing such minute magnitudes is 

 dependent upon long habit and experience. It will be seen from the 

 second report printed below that Dr. R. H. Ward, the well-known micro- 

 scopist, has investigated the matter — under the superintendence of Mr. 

 Fasoldt and his son — and that the results are in accordance with theory. 

 The 110,000 band was seen with perfect ease, and the 120,000 clearly, 

 though with difficulty, " while in higher bands no trace or suspicion of 

 lines was perceived." Mr. Fasoldt himself " did not seem to recognize 

 the lines nearly as far up in the series as this," while his son, who was 

 the manipulator, could see nothing beyond 130,000. Dr. Ward further 

 shows that the people w T ho allege they have seen the higher bands admit 

 that they " furnish only passing glimpses and cannot be kept in focus 

 and examined at leisure or shown to other observers." 



We have prefaced Dr. Ward's report by that of a Mr. P. H. Dudley, 



