138 SUMMARY or CURRENT RESEARCHES RELATING TO 



tion of the screw can be distinctly seen. These waves of light and 

 shade indicate an error which can be seen by the unaided eye but 

 which cannot be measured with certainty. Finally, if the visibility of 

 ruled lines is so erroneously increased by the position which they 

 occupy with respect to the source of light, why may not the visibility 

 under the Microscope be increased in nearly the same proportion by 

 some mechanical device which shall enable the observer to find exactly 

 the proper angle of inclination at which the light should be thrown 

 upon the object in order to secure the best possible result ? " 



Prof. Rogers, in the discussion on a paper by Dr. G. E. Blackham on 

 the Relation of Aperture to Amplification, also said * " The whole thing 

 depends on the question Can we compute resolving powers ? I will 

 not say that we cannot, and I have my doubts if we can. I question 

 the truthfulness of the formula that is used in the computation. My 

 confidence in it was shaken some time ago, when in the measurement 

 of some plates I found errors of 1/40,000 in. I think that the 

 formula is true, so far as it goes, but it does not tell the whole truth. 

 There are conditions that affect it. Take, for instance, Bayard's 

 formula for refractions. It is affected by the atmosphere and temper- 

 ature. Now, I do not say that the two formulas are analogous ; I 

 use Bayard's only as an illustration of what may occur. My position 

 is this : Take what we have as a basis of investigation, and go ahead 

 to ascertain the truth. There is a great sea for exploration in the 

 question." 



Test-Diatoms in Phosphorus and Monobromide of Naphthaline. f 



Canon E. Carr thinks those who are interested in the resolution of 



the more finely marked diatoms, and who have seen or heard of the 

 magnificent image of Surirella gemma, mounted in phosphorus, shown 

 by Mr. J. W. Stephenson at the Society's meetings and conversazioni, 

 with a Zeiss' oil-immersion 1/8 objective and his own catoptric 

 illuminator, will be glad to learn that Moller now supplies some of 

 the more difficult test-objects mounted in highly refractive media. 

 Having recently purchased a slide of Amphipleura pellucida mounted 

 in phosphorus, and one of Surirella gemma mounted in monobromide 

 of naphthaline, he gives the result of his examination of them. The 

 resolution of the hemispherules on the latter was not remarkable, 

 being much the same as that obtained on a slide of the object mounted 

 dry. The resolution of the former, however, was all that could be 

 desired with the means at command, and contrasted favourably with 

 anything he had seen before. Previously, with a Powell and Lealand's 

 water-immersion 1/8 objective, and Wenham disk illuminator, he had 

 seen the striae very faintly shown on a balsam-mounted slide. Much 

 better resolution had been effected on a dry mount by a Powell oil- 

 immersion 1/25 objective, and their achromatic condenser. But even 

 this result was not to be compared with that obtained on the 

 phosphorus mount. Using Powell's oil-immersion 1/12 objective 

 (N.A. 1"43), and their oil-immersion condenser, the strise came out 



* Loc. cit., pp. 227-8. 



t Engl. Mecli., xxxviii. (1883) p. 280. 



