Advantages of the Crescograph.—There is no existing method which enables us to detect and measure such infinitesimal movements and their time-relations. The only attempt made in measuring minute growth has been by observing the movement of a mark on a growing plant through a microscope. The mag­ni­fi­ca­tion available in practice is about 250 times. The observation of the movement would itself be sufficiently fatiguing. But a simultaneous estimate of the time-relations of rapidly fluctuating changes would prove so bewildering, that accurate results from this method would be altogether impossible. A 1/12″ objective gives a linear enlargement of about 1,200 times. But the employment of this objective is impracticable in the measurement of growth elongation of an ordinary plant. With the Crescograph, on the other hand, we obtain a mag­ni­fi­ca­tion which far surpasses the highest powers of a microscope, and it can be used for all plants. It does not merely detect growth but automatically records the rate of growth and its slightest fluctuation. The extreme shortness of time required for an experiment renders the study of the influence of a single factor at a time possible, the other conditions being kept constant. The Crescograph thus opens out a very extensive field of inquiry into the physiology of growth; and the discovery of several important phenomena mentioned in this Paper is to be ascribed to the extreme sensitiveness of the apparatus, and the accuracy of the method employed.

MAGNETIC AMPLIFICATION.

The mag­ni­fi­ca­tion obtained with two levers was, as stated before, 10,000 times. It may be thought that further mag­ni­fi­ca­tion is possible by a compound system of three levers. There is, however, a limit to the number of levers that may be employed with advantage, for the slight overweight of the last lever becomes multiplied and exerts very great tension on the plant, which interferes with the normal rate of its growth. The friction at the bearings also becomes added up by an increase in the number of levers, and this interferes with the uniformity of the movement of the last recording lever. For securing further mag­ni­fi­ca­tion, additional material contact has, therefore, to be abandoned. I have recently been successful in devising an ideal method of mag­ni­fi­ca­tion without contact. The movement of the lever of the Crescograph upsets a very delicately balanced magnetic system. The indicator is a reflected spot of light from a mirror carried by the deflected magnet. Taking a single lever with the lengths of two arms 125 mm. and 2.5 mm. respectively we obtain a mag­ni­fi­ca­tion of 50 times. The magnetic system gives a further mag­ni­fi­ca­tion of 20,000 the total mag­ni­fi­ca­tion being thus a million times. This was verified by moving by means of a micrometer screw the short arm of the lever through 0.005 mm. The resulting deflection of the spot of light at a distance of 4 metres was found to be 5,000 mm. or a million times the movement of the short arm. It is not difficult to produce a further mag­ni­fi­ca­tion of 50 times by attaching a second lever to the first. The total mag­ni­fi­ca­tion would in this case be 50 million times.

A concrete idea of this will be obtained when we realise that by the Magnetic Crescograph a mag­ni­fi­ca­tion can be obtained which is about 50,000 times greater than that produced by the highest power of a microscope. This order of mag­ni­fi­ca­tion would lengthen a wave of sodium light to about 3,000 cm. I am not aware of any existing method by which it is possible to secure an amplification of this order of magnitude. The application of this will undoubtedly be of great help in many physical in­ves­ti­ga­tions, some of which I hope to complete in the near future.

Such an enormous mag­ni­fi­ca­tion cannot be employed in ordinary in­ves­ti­ga­tions on growth, for the moving spot of light indicating rate of growth, passes like a flash across the screen. But it is of signal service in my in­ves­ti­ga­tions on growth by the Method of Balance, to be described in a future Paper. The principle of this method consists in making the spot of light, which is moving in response to growth, stationary, by subjecting the plant to a compensating movement downwards. The slightest variation caused by an external agent would make the spot of light move either to the right or to the left, according to the stimulating or depressing character of the agent. It will be understood, how extremely sensitive this method is for detection of the most minute variation in the normal rate of growth.

THE DEMONSTRATION CRESCOGRAPH.

Before proceeding with accounts of further in­ves­ti­ga­tions, I shall describe a form of Magnetic Crescograph with which I have been able to give before a large audience demonstration of a striking character on various phenomena of growth. The mag­ni­fi­ca­tion obtained was so great that I had to take some trouble in reducing it. This was accomplished by the employment of a single, instead of a compound system of two levers. The reflected spot of light was thrown on a screen placed at a distance of 4 metres, and this gave a mag­ni­fi­ca­tion of a million times; it is obvious that an increase of the distance of the screen to 8 metres would have given a mag­ni­fi­ca­tion of 2 million times. As it was, even the lower mag­ni­fi­ca­tion was far too great for use with quick growing plants like Kysoor. I, therefore, employed the slower growing flower bud of Crinum. It will be seen from [Table X] that the normal rate of growth of the lily is of the order of 0.0006 mm. per second. The normal excursion of the spot of light reflected from the Crescograph exhibit­ing growth was found to be 3 metres in five seconds or 60 cm. per second. This is a million times the actual rate of growth of the Crinum bud. As it is easy to measure 5 mm. in the scale, it will be seen that with the Demonstration Crescograph it is possible to detect the growth of a plant for a period shorter than a hundredth part of a second.

Experiment 57.—A scale 3 metres long divided into cm. is placed against the screen. A metronome beating half seconds is started at the moment when the spot of light transits across the zero division; the number of beats is counted till the index traverses the 300 cm. At the normal temperature of the room (30°C.), the index traversed 300 cm. in five seconds. The plant chamber was next cooled to 26°C. by the blowing in of cooled water vapour; the time taken by the spot of light to traverse the scale was now 20 seconds, i.e., the growth-rate was depressed to a fourth. Under continuous lowering of temperature the growth-rate became slowed down till at 21°C. there was an arrest of growth. Warm vapour was next introduced, gradually raising the temperature of the chamber to 35°C. The spot of light now rushed across the scale in a second and a half, i.e., the growth was enhanced to more than three times the normal rate. The entire series of the above experiments, on the effect of temperature on growth, was thus completed in the course of 15 minutes.

SUMMARY.