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A crystal X-ray spectrometer selects a narrow wavelength interval by diffraction at known interplanar spacing $d$ and Bragg angle $\theta$. For fixed diffraction order $n$, a known $d$ lets an angular measurement determine $\lambda =2d\sin \theta /n$. This reverses the crystallography problem of finding $d$ from a known wavelength.

Siegbahn's laboratory developed X-ray tubes and evacuated spectrographs for longer wavelengths. Rotating the crystal selected reflections; photographic plates recorded positions. Opposite-side exposures measured angles on both sides of the direct beam. Later tube instruments mounted a beam-defining slit opposite the plate holder. A separate double instrument compared two reflected beams with two ionization chambers. These were different instruments, not one universal assembly.^[ref]

The Nobel biography describes constant-temperature spectrographs and a precision angular scale read by diametrically opposed microscopes.^[ref] Temperature control matters because thermal expansion changes $d$; an accurately read angle cannot compensate for an unknown interplanar spacing.

The schematic separates preparation, selection and readout. An X-ray tube target supplies Characteristic X-rays with a possible bremsstrahlung background. A collimator limits incident directions; evacuation reduces air X-ray attenuation. Rotating the crystal brings different wavelengths into reflection. A photographic plate stores line positions over an exposure, whereas an ionization chamber can measure collected electric charge at successive angles. Neither readout alone establishes absolute wavelength: an angular zero, angular-scale calibration, diffraction order, $d$ and any required refraction correction enter the inference. Narrower slits reduce accepted angular spread but also reduce signal; a weak line can become harder to locate despite a narrower instrumental response.

A control reported in Siegbahn's lecture photographed a line system with copper-sulphate and calcium-sulphate crystals. Irregular relative shifts around copper's characteristic frequency supported anomalous dispersion; the arrangement excluded differing absorption across that frequency as the explanation.^[ref] A finer angular scale alone cannot remove this physical refraction correction.

Wilhelm Conrad Röntgen's 1895 first communication describes an induction coil driving an evacuated gas-discharge tube, enclosed in black cardboard in darkness. A paper screen coated with barium platinocyanide showed fluorescence even with its uncoated side facing the tube, visible two metres away. The covering excluded ordinary illumination.^[ref]

He interposed materials: a thousand-page book transmitted; $1.5\mathrm{mm}$ lead was practically opaque. A stepped stack of foil produced a photographic plate record. Different materials were not equivalent at equal density and thickness. Photographic plates checked visual observations while remaining covered against ordinary illumination.^[ref]

Section 14 lists photographs of hand bones, wire on a bobbin, boxed weights, an enclosed compass, and a lead-painted door. It does not identify the hand's owner. Magnetic deflection moved the cathode ray impact point and thus the source, without bending the emerging X-rays. Their nature remained unsettled.^[ref]

The observations belong to 1895; the contemporary English translation appeared in Nature on 23 January 1896.^[ref] The experimental logic separates production, transmission and detection. A visibly opaque covering can transmit X-rays; fluorescence then converts deposited energy into visible light. A shadow measures a difference between paths through an object, rather than a direct view of its surface. Neither a screen's brightness nor photographic darkness alone measures an absolute X-ray energy or establishes the X-ray attenuation law below.