324 SUMMARY OF CURRENT RESEARCHES RELATING TO 



higher bands of Nobert's 19-band test plate, were obtained by using sun-&| 

 light. _ ^ 



The main purpose of his visit to Jena, however, was to submit to 

 Prof. Abbe's examination a number of the best English objectives, whence 

 he could accurately estimate the standpoint of excellence from which 

 English microscopists would criticize the new apochromatics produced at 

 Jena. In furtherance of this jjurpose the President of the Society and 

 Mr. Frank Crisp had placed at his disposal the best objectives in their 

 collections. Mr. Nelson had also requested him to select from his fine 

 collection any objectives which he thought would worthily represent English 

 optical work. From these collections, and sundry examples from his own, 

 Mr. Mayall said he believed he had been able to carry out the intention 

 of his visit to Jena ; and he thought Prof. Abbe was now as vividly aware 

 of what was meant in England by " critically good images " as possibly 

 could obtain under the circumstances. He must, of course, mention the 

 fact that he took with him to Jena his large Powell and Lealand Micro- 

 scope and accessory apparatus. If his visit to Jena resulted in inducing 

 Prof. Abbe to withdraw his frequently-expressed depreciation of the value 

 of the achromatic condenser — and he had reason to believe this would be 

 one of the practical results following upon his visit — he (Mr. Mayall) 

 should consider his journey not wholly fruitless in advancing practical 

 Microscopy. 



Eeferriug to the Jena Optical Glass Works, Mr. Mayall said they were 

 under the management of Dr. Otto Schott, who appeared to have thrown 

 his energy thoroughly into every detail of their organization, which had so 

 favourably impressed the German Government that large ofiicial grants of 

 money had been made in aid of the experiments suggested by him. The 

 aim of the series of experiments had been to arrive at a knowledge of the 

 conditions necessary for regulating the refractive and dispersive indices as 

 far as possible with the various known substances capable of vitrification. He 

 understood Dr. Schott to say the experience he had gained in the experi- 

 ments made with the assistance of the Government — experiments which 

 had all been carefully classified and I'ecorded — enabled him now to under- 

 take to furnish any kind of optical glass according to sample supplied to 

 him. On receiving such a sample, he proceeded to analyse it both optically 

 and chemically, and then, from his registrations of experiments already 

 made, he was able at once to select the elements and conditions required to 

 arrive at the same result. Moreover, the exhaustive series of experiments 

 he had made, enabled him, within certain limits, to control the ratio of the 

 refraction to the dispersion, so that he had not only succeeded in increasing 

 the range between the limits beyond what had been reached previously 

 by makers of optical glass, but was also in a position to manufacture glass 

 of any given refraction and dispersion for special purposes. The skilful 

 optician was thus provided with new optical means which would certainly 

 lead to general improvements in the construction of telescopes, field- 

 glasses, &c. The new kinds of glass employed in Prof. Abbe's apochro- 

 matic objectives were produced at these Glass Works, as also the glass 

 employed by Messrs. Powell and Lealand for their new apochromatics. 

 Dr. Schott expressed his conviction that several of his new kinds of glass 

 would be found of great importance in the construction of photographic 

 lenses ; he also said that Steinheil, the well-known optician of Munich, had 

 already adopted its use largely. Such a fact ought not in his (Mr. Mayall's) 

 opinion to be neglected by our makers of photographic lenses ; for, 

 assuredly, if one of them could succeed in producing lenses with a given 

 ratio of aperture to focal length, but with a larger and flatter field than 



