The timing of pre-mRNA splicing visualized in real-time.Since it became clear that intervening sequences or introns are spliced out from precursor pre-mRNA molecules in the nucleus before mature mRNAs are exported to the cytoplasm, questions were raised about the timing of splicing. 
The timing of pre-mRNA splicing visualized in real-time
Experiments done with the electron microscope paved the way to the discovery of splicing and provided unequivocal evidence that splicing can occur co-transcriptionally. 
electron microscope
More recently, live-cell microscopy introduced a technical breakthrough that allows real-time visualization of splicing dynamics. 
live-cell microscopy