Understanding the Physical World

Lecture delivered in Hamman Hall on February 21, 1962, as the first of the Alumni Association's Distinguished Scholar Progra

A spectroscopic determination of e/m

Rydberg constants for hydrogen and helium.—By an interferometer method the wave-lengths of the hydrogen lines at 6563A and 4861A were measured with reference to λ5015.6750 of He as standard. The values so obtained were, for the doublet at 6563A, 6562.7110±.0018 and 6562.8473±.0009; and for the doublet at 4861, 4861.2800±.0013 and 4861.3578±0022. Similarly the wave-lengths of the 4686 lines of ionized helium were found to be 4685.7030±.0012 and 4685.8030±.0026. From these values of the wave-lengths the Rydberg constant for hydrogen, RH is calculated to be 109,677.759±.008, and the Rydberg constant for helium, RHe, to be 109, 722.403±.004. Calculation of e/m.—The ratio of the mass of the electron to the mass of the hydrogen nucleus, m/mH, is given by m/mH=(RHe-RH)(mHe-m)/RH(mHe-mH)=1.33648(RHe-RH)/RH. The value of e/m is given in terms of the Faraday constant as e/m=(mH/m)(F/mH). If the values of RH and RHe calculated above be substituted in these expressions and if it be assumed that F=96470 abamp. per gr. mol. and mH=1.0072, it is found that e/m has the value (1.7606±.0010)×10^(7) e.m.u. per gr

Some relationships between singlets and triplets in the spectra of two electron systems

The Darwin-Pauli treatment of the electron is applied to the Schroedinger equation for a two electron system. The results show that the division into singlets and triplets is justified only as a limiting case. Expressions are derived which give the position of the levels, the Zeeman effect pattern, and the intensities of the lines when the division into singlet and triplet cannot strictly be made. These expressions are shown to give the observed facts of several spectra

A compound interferometer for fine structure work

The overlapping of orders in a Fabry-Perot interferometer can be avoided by using two interferometers in series. The fundamental equation shows that the dispersion is independent of the plate separation, while the distance between orders is inversely proportional to it. Thus an instrument with a small separation may be used as a preliminary filter to eliminate some of the orders in one of larger separation. This will not affect the fine structure pattern, and the resolution will be even greater than that due to the larger separation. Such an instrument has been built and satisfies the predictions of the theory. A plate, taken with the green line of mercury, is shown to illustrate the effect of the preliminary interferometer

The temperature dependence of electron emission under high fields

An expression showing the temperature dependence of electron emission under high fields is secured by combining the results of Fowler and Nordheim with the Fermi distribution of velocities used in the Sommerfeld electron theory of metals. The result is similar to that obtained previously by considering the diminution of the work function by the field. The temperature variation is small and decreases as the external field increases. It is of the right order of magnitude to agree with the most recent observations

The temperature dependence of electrical conductivity

To explain the fact that the resistance of a pure metal becomes zero at the absolute zero of temperature it is necessary to take into account the restrictions placed upon the electron scattering by the quantum statistics. When these are included as a probability of transition, the Brillouin treatment of wave scattering gives a very satisfactory law of the temperature dependence of resistance

Set Pseudophasors to Stun for Flow Cytometry

Study of signal transduction in live cells benefits from the ability to visualize and quantify light emitted by fluorescent proteins (XFPs) fused to different signaling proteins. However, because cell signaling proteins are often present in small numbers, and because the XFPs themselves are poor fluorophores, the amount of emitted light, and the observable signal in these studies, is often small. An XFP's fluorescence lifetime contains additional information about the immediate environment of the fluorophore that can augment the information from its weak light signal. Here, we constructed and expressed in Saccharomyces cerevisiae variants of Teal Fluorescent Protein (TFP) and Citrine that were isospectral but had shorter fluorescence lifetimes, ∼ 1.5 ns vs ∼ 3 ns. We modified microscopic and flow cytometric instruments to measure fluorescence lifetimes in live cells. We developed digital hardware and a measure of lifetime called a "pseudophasor" that we could compute quickly enough to permit sorting by lifetime in flow. We used these abilities to sort mixtures of cells expressing TFP and the short-lifetime TFP variant into subpopulations that were respectively 97% and 94% pure. This work demonstrates the feasibility of using information about fluorescence lifetime to help quantify cell signaling in living cells at the high throughput provided by flow cytometry. Moreover, it demonstrates the feasibility of isolating and recovering subpopulations of cells with different XFP lifetimes for subsequent experimentation

Epigenetic regulation during cancer transitions across 11 tumour types

Chromatin accessibility is essential in regulating gene expression and cellular identity, and alterations in accessibility have been implicated in driving cancer initiation, progression and metastasi