MEMORANDA. 281 



Heuriscopometer.— Those who study the animalcules, and who 

 make i-esearches among the diatoms or other microscopical 

 shells as a matter of preference, experience great difficulties in 

 exploring a preparation which often contains several millions 

 of these little creatures, each of which has a siliceous carapace, 

 and which have played such an important part in the earth's 

 phenomena of the tertiary epoch. The difficulty is much greater 

 still when it is necessary for them to refind in a considerable 

 number of individuals those which particularly attracted their 

 attention at the time of a first examination. It sometimes 

 happens that, after several hours of research, they cannot 

 attain it, and if patience is not wanting to them, fatigue, at 

 least, obliges them momentarily to relax their labours. Not 

 to refind what one has already seen in a preparation which 

 can scarcely be a centimetre in diameter will doubtless 

 appear extraordinary to those who are strangers to micro- 

 scopical studies. Whilst the smaller the animals one exa- 

 mines the greater ought the magnifying power of the 

 microscope to be, it is certain that the field of the instrument 

 diminishes in proportion as the extent of the preparation in- 

 creases. With a magnifying power of 2000 diameters, for 

 instance, a preparation of one centimetre square will attain, 

 then, a superficies of twenty centimetres on each side. Every 

 one will comprehend the difficulty of finding in so large a 

 space, of which the field of the microscope occupies but a 

 very small part, the little being which at first attracted atten- 

 tion, whether on account of its peculiar formation, or by certain 

 characteristics indicating in the individual a new species 

 which it is necessary to classify. To obviate this inconve- 

 nience several methods have been used. In 1855 Professor 

 T. W. Bailey, of the United States, proposed a universal 

 indicator ; it was not really an instrument, for it consisted 

 but of a divided card that was placed on the stage of the 

 microscope, and which offered, as one may suppose, no 

 guarantee for the exactitude of the researches. The one 

 lately indicated by Mr. Wright, in the ' Microscopical 

 Journal,' was not more practical. I sent to the London 

 Universal Exhibition, in 1863, a metal indicator of a very 

 simple construction, depending on a geometrical principle, 

 and being adaptable to all microscopes. It was entered in 

 the general catalogue. No. 1419, in the 13th class. This 

 instrument was very simple ; in fact, one of its movements is 

 regulated by a micrometric vice, the other by the fingers only. 

 Thi^ indicator, once placed on the stage of the microscope in 

 a fixed and invariable position, the object is refound by the help 

 of the co-ordinates, of which the figures have been written 



