PROaRESS OF MICROSCOPICAL SCIENCE. 275 



seen with a power of 125, and countetl witli a power of 160 (the 

 distance for the unaided eye being considered 8 inches). We have, 

 however, obviously in the high bands to inchide the case of obsei-va- 

 tion 7, the lines on the photographs being excessively faint. When 

 we add to this fact (a most important one when such lines are sup- 

 posed to give some measure of the power of the microscope) that it 

 appears that separate lines cannot be drawn of a less width than 

 about Y^^Voo^ ^^ *^ inch under the diminished pressure of Mr. 

 Nobert's machine without the graving-point sliding into previous 

 grooves, we have a sufficient explanation why the power of the micro- 

 scope cannot be measured by these lines. 



The following are the conclusions of this note : 



1st. That lines can be seen by the naked eye with transmitted 

 light the width of which subtends an angle of about 1". 



2nd. That the visibility of a line, or the distance at which it can 

 be seen, depends on the logarithm of its length, the product of the 

 angle subtended by the width and the cube root of that subtended by 

 the length being nearly constant. 



3rd. Short parallel lines could be seen by transmitted light when 

 the angle formed by the width of the spaces and intervals was 20". 



4th. The visibility of lines of the same width increases as the 

 distance between them decreases, 



5th, The visibility of parallel lines depends on the darkness of the 

 shade or tint of the lines up to a certain feeble tint, after which no 

 blacking of the lines increases the visibility ; the distance to which 

 the lines can be seen depends on c', where c is a constant and t is the 

 number of the tint or shade (the number of coats of a weak tint). 



6th, The visibility of dark parallel lines lighted with a candle 

 depends on the logarithm of the distance of the candle from the lines ; 

 and they can be seen as well with a candle placed quite near as with 

 the strongest daylight. This results from Tobias Mayer's observations. 



7th. The visibility of parallel lines depends on the logarithm 

 of their length, as in the case of single lines, the variation being 

 much greater for short parallel lines than for long ones. Also for 

 short parallel lines the product a IJ (3 is nearly constant, as for single 

 lines, 



8th, Parallel lines are least visible when there are only two, and 

 increase in visibility with their number, 



9th, Nobert's test-lines fail as a test for the microscope, especially 

 in the highest bands, from the incapacity of the machine to make 

 separate lines at less intervals and of less width than y^oWo ^^ ^^ 

 inch ; they also fail, in all probability, on account of the faintness of 

 the tint or shade of the lines made on the retina. 



A peculiar Process of Development in certain Fungi has been 

 recently recorded before the French Academy, and has been well ab- 

 stracted by the ' Academy,' which says that M. Ph, van Tieghem has 

 made fresh observations on the development of agarics of the genus 

 Coprinus, and on the supposed sexuality of Basidiomycetes, which differ 

 from the conclusions lately arrived at. He finds that certain rods 

 (hatonnet^) which appeared to be male organs, and to act as fecun- 



