This book has charted the astonishing leaps that forensic science has made over the last two hundred years. If we presented Michael Faraday or Paracelsus with the scientific evidence our courts now take for granted, it would seem like magic to those most rigorous of researchers. And the advance of science has run hand in hand with corresponding advances in the delivery of justice.

When beat officer John Neil arrived at the scene of the first Jack the Ripper murder in 1888, he faced insurmountable problems. No one in Whitechapel's intricate network of alleyways and streets had seen the murderer on that August night. There was no obvious motive, and no obvious suspect. Mary Nichols's body offered evidence about the murder weapon, about the strength of the murderer himself and the state of his twisted mind. But none of this pointed in any decisive direction.

Had Neil and his colleagues had the skills and technology of modern forensic investigators, processing the scene would almost certainly have led them to follow Holmes' 'scarlet thread of murder' inexorably to the man who killed those Whitechapel women in the dead of night. But without the most basic of scientific resources, the police were fumbling in the dark. They knew it, and the public knew it: a popular cartoon of the time showed a blindfolded officer stumbling hopelessly around in a street full of Rippers laughing and goading him.

The five acknowledged victims of the Ripper were Mary Ann Nichols,

Annie Chapman, Elizabeth Stride, Catherine Eddowes and Mary Jane Kelly. They represent a tiny proportion of the men, women and children whose killers have escaped retribution, simply because there was no way to unravel the complex circumstances of a murder scene. But the police and forensic services have learned lessons from these failures that ultimately have served to protect others. Even the several thousand dogs who died slow deaths by poison at the hands of the 'father of toxicology', Mathieu Orfila, in the early 1800s, had a significant role to play.

In the course of researching this book, I have been struck, above all, by the integrity, ingenuity and generosity of the forensic scientists I have met. They care so deeply about the cases they work on that they are willing to engage with the darkest and most frightening aspects of human behaviour on a daily basis. They are willing, like Niamh Nic Daéid, to spend hours in the sodden debris of a fatal fire; like Martin Hall, to collect maggots from a week-old corpse; or, like Caroline Wilkinson, to reconstruct the face of a mutilated child the same age as her own daughter. They make sacrifices so that the rest of us can live knowing that, if we are the victims of crime, the perpetrators will be brought to justice. They do not guard their knowledge jealously; they share it as widely as possible in the hope that one of their colleagues may use it as a springboard for the next leap forward.

And the importance of their work makes them astonishingly creative in the face of a tricky forensic problem. The proliferation of forensic tools made available to crime investigators over the last two hundred years is nothing short of astounding. And, although they are all imperfect, nearly all have strengthened the criminal justice system. We've heard about the 'bucket science' that characterised the early days of DNA analysis; now a scientist like Val Tomlinson or Gill Tully can use a bloodstain a millionth the size of a grain of salt to provide a profile that can find not only the person it belongs to, but also a member of their family who might have committed a crime, maybe years ago. Confronted with a video that appeared to show sexual abuse but not the abuser's face, Sue Black became the first person to identify someone from the unique pattern of veins on their forearms and freckles on their hand. These scientists find their imaginations stimulated rather than curbed by the challenges of crime investigation – and by the need to be rigorous.

Crime scene evidence would not be so effectively utilised today if it hadn't, for more than two hundred years, been forced to pass the strict credibility tests of the courtroom. The first pressure is put on a scientist's theory by their scientific peers, who force them either to abandon it or meet the challenge and make it stronger. Then, in the courtroom, lawyers do everything they can to excite scepticism in the jury. Very few holds are barred on the witness stand, and a lawyer may choose to ignore their scientific methods and interrogate their character instead. But, however personally stressful a forensic scientist may find giving testimony, the courtroom is the anvil on which scientific evidence is struck. With a wellprepared lawyer playing the part of the hammer, forensic techniques are either strengthened or broken, according to their merit.

Of course, as parts of this book have shown, it doesn't always run like clockwork. But when it does, inspirational sparks fly, new ideas are knocked out and the room for manoeuvre enjoyed by violent criminals shrinks a little more.

The methods of science and justice have much in common. Both attempt to shine a clarifying light on obscurity and uncertainty. At best, their core aims match, too, as they try to go beyond assumption and arrive at the truth though demonstrable facts. Yet because forensic science is made up of so many human layers – criminals, eyewitnesses, police officers, CSIs, scientists, lawyers, judges, juries – it cannot avoid either missing or misrepresenting the truth at times. The stakes are always high; life and liberty depend on it. I hope this book has demonstrated the commitment of forensic scientists across the disciplines to be imaginative, open-minded and painstakingly honest in the interests of justice for all of us. It has certainly reminded me of what I have known for a long time – the work itself is amazing and the people who do it are, frankly, awesome.

1 Crime scene notes taken by John Glaister Junior, the leading forensic investigator in the Buck

Ruxton case 

2, 3 Police officers comb the area where the remains of Isabella Ruxton and her maid, Mary Rogerson, were found. The bodies were recovered in over thirty separate packages, leading many to call the case the 'Jigsaw Murders'

4 A maggot's head under a microscope. Note the two prongs, used for scraping decaying flesh into its mouth

5 Blowfly (Sarcophaga nodosa) on decaying flesh. Blowflies can smell decomposition from over on hundred metres, making them 'the gold standard indicators' of the insect world 

6 An illustration from Eduard Piotrowski's seminal work on bloodstains; as part of his research, he bludgeoned animals with a variety of instruments to observe the effects

7 At the University of Tennessee's 'Body Farm', bodies like this one are left to decompose in a variety of different settings for the purposes of study. This image is part of photographer Sally Mann's series 'What Remains'. Sally Mann, 'Untitled', 2000, gelatin silver print, 30 x 38 inches, edition of three

8, 9, 10 Graham Coutts, who was convicted of Jane Longhurst's murder, caught on CCTV moving her body from the storage facility where he kept it in the weeks after her death

11 Death of a court lady, from a series of eighteenth-century Japanese watercolours depicting the nine stages of a decaying corpse or kusōzu: the putrefying body is carrion for scavenging birds and small animals;

12 at this stage the flesh has almost all decayed revealing the skeleton. Wistaria blossoms above her body;

13 only a few fragments of bone, including the skull and ribs, hand and vertebrae, remaiN

14, 15 Betty P. Gatliff (see p.198) working on a facial reconstruction of one of serial killer John W. Gacy, Jr.'s nine unidentified victims in July 1980. Photographs of the reconstructed heads were released to the media in an attempt to identify the victims. To her right are a completed reconstruction and a skull with the rubber guides which show the average thickness of tissue on a human face

16 Sections of the brain of a gunshot victim, showing the path of the bullet, and (to right) the bullet itself

17 A section from a liver, and (to left) the knife that caused the fatal wound

18 One of Frances Glessner Lee's doll's house-sized 'Nutshell Studies of Unexplained Death'. Designed to help train police recruits in detection, the

Nutshell Studies depict imaginary crime scenes down to the tiniest details

19 A model of an old man's head in wax, created by the seventeeth-century sculptor Giulio Zumbo. Zumbo created many detailed anatomical models; in this case he built up layers of coloured wax onto a real skull