CORRESPONDENCE. 241 



focus (the latter limited to a very small angle), and the objects should 

 be ijlaced in a jar or tank having the bottom and at least one side quite 

 smooth and transparent. 



How to study Embryology. — This is well shown by the recent 

 observations of Dr. Elias Metschnikoff upon the development of scor- 

 l^ions. This author has recently published in Siebold and Kolliker's 

 ' Zeitschrift,' a paper which is abstracted by the 'American Naturalist,' 

 The subject is the embryology of the Scorpio Italicus and of a species 

 from Tyrol. The embryology of insects and Crustacea as i)ursued at 

 the present day by zoologists, who are directing esj)ecial attention to 

 the provisional membranes of the egg and embryo, depends almost as 

 much on the skilful use of the chemicals as the microscope itself. The 

 author says " the methods which I employ in these researches are not 

 complicated. I study the eggs removed from the ovarian tubes ; or, 

 place the living embryo in a drop of a weak solution of salt (salzlosung) ; 

 or I at first submit them to the influence of solutions of chromic acid 

 of different strengths, and then examine them either with a simple or 

 compound microscope. Out of embryos hardened in this way I can 

 make sections. Much of the time I have to work with dissecting 

 needles, while the embryos or portions of them treated in this way, 

 and in an equal mixture of fresh and salt water, afford very good 

 objects for study." The embryology of scorpions was sketched out in 

 a general way by the distinguished German embryologist Eathke. 

 Metschnikoff' extends these researches very greatly, and considers as 

 the most important results of his studies the discovery that " in the 

 embryo of the scorpions three embryonal membranes are develojied, 

 which in many respects are very strikingly similar to the Eemakian 

 embryonal membranes of the vertebrates." 



COEKESPONDENCE. 



What is the Aperture of Tolles' ^th? 



To the Editor of the ' Monthly Microscopical Journal.' 



Sept. l^th. 



Sir, — In the very interesting account given by Colonel Woodward 

 of Mr. Tolles' fifth there is one important omission. He has not told 

 us what is its aperture. As a general rule a one-fifth by any of the 

 best of our English makers does not exceed 100^ in aperture ; but it 

 would not " sm'prise me to hear " that Mr. Tolles' fifth has an ajierture 

 of 150° or 160°. At the close of his paper it is suggested that trials 

 should be made with recent English glasses of the same focus, so as 

 to compare the results with those obtained with Mr. Tolles' ; but 

 until the respective apertures are known, such comparisons would be 

 without meaning. Perhaps Colonel Woodward, if this should meet 

 his eye, will be kind enough to supply the omission. 



As I am writing on the subject, I may take the opportunity to 

 point out more generally the confusion which exists in the minds of 



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